refactor the CI (#9626)

# Description
this PR is an attempt at making the main CI workflow a bit easier to
read, understand and work with 🤞

## changelog
- remove the deprecated `nu-coverage` job which was taking place for
nothing
- remove useless comments, i.e. comments that just say the same thing as
the line they comment, e.g. the previous `on.push` comment, and comments
that can be explained with a `git blame` inside an IDE, e.g. the comment
about removing `--profile ci` on the `cargo install` in the std job
- rename `nu-fmt-clippy` and `nu-tests` to `fmt-clippy` and `tests` as
we already know we are working with Nushell
- rename the "*style*" to "*feature*" which is exactly what `default`,
`dataframe` and `extra` are
- remove the `strategy.matrix.rust: stable` key: i couldn't find any
documentation for this and it's not used in the jobs => by default, the
toolchain of `actions-rust-lang/setup-rust-toolchain@v1.5.0` is the
stable one (see the
[documentation](https://github.com/actions-rust-lang/setup-rust-toolchain#inputs))
- add a top-level `env` block with all the environment variables, e.g.
`CLIPPY_OPTIONS`, to have them all in one place

hope you'll like that 😇 🙏 

# User-Facing Changes

# Tests + Formatting
this PR should not change the different jobs being run in PRs and on the
`main` branch.

# After Submitting

.github/workflows/ci.yml

@@ -1,13 +1,18 @@
 on:
   pull_request:
-  push: # Run CI on the main branch after every merge. This is important to fill the GitHub Actions cache in a way that pull requests can see it
+  push:
     branches:
       - main
 
 name: continuous-integration
 
+env:
+  NUSHELL_CARGO_TARGET: ci
+  NU_LOG_LEVEL: DEBUG
+  CLIPPY_OPTIONS: "-D warnings -D clippy::unwrap_used -A clippy::needless_collect -A clippy::result_large_err"
+
 jobs:
-  nu-fmt-clippy:
+  fmt-clippy:
     strategy:
       fail-fast: true
       matrix:
@@ -15,28 +20,21 @@ jobs:
         # builds to link against a too-new-for-many-Linux-installs glibc version. Consider
         # revisiting this when 20.04 is closer to EOL (April 2025)
         platform: [windows-latest, macos-latest, ubuntu-20.04]
-        style: [default, dataframe, extra]
+        feature: [default, dataframe, extra]
-        rust:
-          - stable
         include:
-          - style: default
+          - feature: default
             flags: ""
-          - style: dataframe
+          - feature: dataframe
-            flags: "--features=dataframe "
+            flags: "--features=dataframe"
-          - style: extra
+          - feature: extra
-            flags: "--features=extra "
+            flags: "--features=extra"
         exclude:
-          # only test dataframes on Ubuntu (the fastest platform)
           - platform: windows-latest
-            style: dataframe
+            feature: dataframe
           - platform: macos-latest
-            style: dataframe
+            feature: dataframe
-
-
 
     runs-on: ${{ matrix.platform }}
-    env:
-      NUSHELL_CARGO_TARGET: ci
 
     steps:
       - uses: actions/checkout@v3
@@ -50,39 +48,30 @@ jobs:
         run: cargo fmt --all -- --check
 
       - name: Clippy
-        run: cargo clippy --workspace ${{ matrix.flags }}--exclude nu_plugin_* -- -D warnings -D clippy::unwrap_used -A clippy::needless_collect -A clippy::result_large_err
+        run: cargo clippy --workspace ${{ matrix.flags }} --exclude nu_plugin_* -- $CLIPPY_OPTIONS
-
-  nu-tests:
-    env:
-      NUSHELL_CARGO_TARGET: ci
 
+  tests:
     strategy:
       fail-fast: true
       matrix:
         platform: [windows-latest, macos-latest, ubuntu-20.04]
-        style: [default, dataframe, extra]
+        feature: [default, dataframe, extra]
-        rust:
-          - stable
         include:
-          - style: default
+          - feature: default
             flags: ""
-          - style: dataframe
+          - feature: dataframe
             flags: "--features=dataframe"
-          - style: extra
+          - feature: extra
             flags: "--features=extra"
-
         exclude:
-          # only test dataframes and extra on Ubuntu (the fastest platform)
           - platform: windows-latest
-            style: dataframe
+            feature: dataframe
           - platform: macos-latest
-            style: dataframe
+            feature: dataframe
           - platform: windows-latest
-            style: extra
+            feature: extra
           - platform: macos-latest
-            style: extra
+            feature: extra
-
-
 
     runs-on: ${{ matrix.platform }}
 
@@ -98,15 +87,10 @@ jobs:
         run: cargo test --workspace --profile ci --exclude nu_plugin_* ${{ matrix.flags }}
 
   std-lib-and-python-virtualenv:
-    env:
-      NU_LOG_LEVEL: DEBUG
-
     strategy:
       fail-fast: true
       matrix:
         platform: [ubuntu-20.04, macos-latest, windows-latest]
-        rust:
-          - stable
         py:
           - py
 
@@ -121,15 +105,6 @@ jobs:
           rustflags: ""
 
       - name: Install Nushell
-        # prior to [*standard library: bring the tests into the main CI*](#8525)
-        # there was a `--profile ci` here in the `cargo install`, as well as
-        # `NUSHELL_CARGO_TARGET: ci` in the prelude above.
-        #
-        # this caused a "stackoverflow" error in the CI on windows,
-        # see [this failing job](https://github.com/nushell/nushell/actions/runs/4512034615/jobs/7944945590)
-        #
-        # the CI profile has been removed in 00b820de9021227d1910a9ea388297ee7aee308e
-        # as part of #8525.
         run: cargo install --path . --locked --no-default-features
 
       - name: Standard library tests
@@ -142,7 +117,6 @@ jobs:
 
       - run: python -m pip install tox
 
-        # Get only the latest tagged version for stability reasons
       - name: Install virtualenv
         run: git clone https://github.com/pypa/virtualenv.git
         shell: bash
@@ -158,18 +132,11 @@ jobs:
           tox -e ${{ matrix.py }} -- -k nushell
         shell: bash
 
-  # Build+test plugins on their own, without the rest of Nu. This helps with CI parallelization and
-  # also helps test that the plugins build without any feature unification shenanigans
   plugins:
-    env:
-      NUSHELL_CARGO_TARGET: ci
-
     strategy:
       fail-fast: true
       matrix:
         platform: [windows-latest, macos-latest, ubuntu-20.04]
-        rust:
-          - stable
 
     runs-on: ${{ matrix.platform }}
 
@@ -182,49 +149,7 @@ jobs:
           rustflags: ""
 
       - name: Clippy
-        run: cargo clippy --package nu_plugin_* ${{ matrix.flags }} -- -D warnings -D clippy::unwrap_used -A clippy::needless_collect -A clippy::result_large_err
+        run: cargo clippy --package nu_plugin_* ${{ matrix.flags }} -- $CLIPPY_OPTIONS
 
       - name: Tests
         run: cargo test --profile ci --package nu_plugin_*
-
-
-  nu-coverage:
-    if: false
-    env:
-      NUSHELL_CARGO_TARGET: ci
-
-    strategy:
-      fail-fast: true
-      matrix:
-        # disabled mac due to problems with merging coverage and similarity to linux
-        # disabled windows due to running out of disk space when having too many crates or tests
-        platform: [ubuntu-20.04] # windows-latest
-        rust:
-          - stable
-
-    runs-on: ${{ matrix.platform }}
-
-    steps:
-      - uses: actions/checkout@v3
-
-      - name: Setup Rust toolchain and cache
-        uses: actions-rust-lang/setup-rust-toolchain@v1.5.0
-        with:
-          rustflags: ""
-
-      - name: Install cargo-llvm-cov
-        uses: taiki-e/install-action@cargo-llvm-cov
-
-      - name: Tests
-        shell: bash
-        run: |
-          source <(cargo llvm-cov show-env --export-prefix) # Set the environment variables needed to get coverage.
-          cargo llvm-cov clean --workspace # Remove artifacts that may affect the coverage results.
-          cargo build --workspace --profile ci
-          cargo test --workspace --profile ci
-          cargo llvm-cov report --profile ci --lcov --output-path lcov.info
-
-      - name: Upload coverage reports to Codecov with GitHub Action
-        uses: codecov/codecov-action@v3
-        with:
-          files: lcov.info