remove let-env, focus on mutating $env (#9574)

# Description

For years, Nushell has used `let-env` to set a single environment
variable. As our work on scoping continued, we refined what it meant for
a variable to be in scope using `let` but never updated how `let-env`
would work. Instead, `let-env` confusingly created mutations to the
command's copy of `$env`.

So, to help fix the mental model and point people to the right way of
thinking about what changing the environment means, this PR removes
`let-env` to encourage people to think of it as updating the command's
environment variable via mutation.

Before:

```
let-env FOO = "BAR"
```

Now:

```
$env.FOO = "BAR"
```

It's also a good reminder that the environment owned by the command is
in the `$env` variable rather than global like it is in other shells.

# User-Facing Changes

BREAKING CHANGE BREAKING CHANGE

This completely removes `let-env FOO = "BAR"` so that we can focus on
`$env.FOO = "BAR"`.

# Tests + Formatting
<!--
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Make sure you've run and fixed any issues with these commands:

- `cargo fmt --all -- --check` to check standard code formatting (`cargo
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you're using the standard code style
- `cargo test --workspace` to check that all tests pass
- `cargo run -- crates/nu-std/tests/run.nu` to run the tests for the
standard library

> **Note**
> from `nushell` you can also use the `toolkit` as follows
> ```bash
> use toolkit.nu # or use an `env_change` hook to activate it
automatically
> toolkit check pr
> ```
-->

# After / Before Submitting
integration scripts to update:
- ✔️
[starship](https://github.com/starship/starship/blob/master/src/init/starship.nu)
- ✔️
[virtualenv](https://github.com/pypa/virtualenv/blob/main/src/virtualenv/activation/nushell/activate.nu)
- ✔️
[atuin](https://github.com/ellie/atuin/blob/main/atuin/src/shell/atuin.nu)
(PR: https://github.com/ellie/atuin/pull/1080)
- ❌
[zoxide](https://github.com/ajeetdsouza/zoxide/blob/main/templates/nushell.txt)
(PR: https://github.com/ajeetdsouza/zoxide/pull/587)
- ✔️
[oh-my-posh](https://github.com/JanDeDobbeleer/oh-my-posh/blob/main/src/shell/scripts/omp.nu)
(pr: https://github.com/JanDeDobbeleer/oh-my-posh/pull/4011)

tests/parsing/mod.rs

@@ -79,7 +79,7 @@ fn parse_file_relative_to_parsed_file_simple() {
                 r#"
                     use ../lol_shell.nu
 
-                    let-env LOL = (lol_shell ls)
+                    $env.LOL = (lol_shell ls)
                 "#,
             )])
             .with_files(vec![FileWithContentToBeTrimmed(
@@ -115,7 +115,7 @@ fn parse_file_relative_to_parsed_file() {
                     use ../lol_shell.nu
                     overlay use ../../lol/lol_shell.nu
 
-                    let-env LOL = $'($env.FOO) (lol_shell ls) (ls)'
+                    $env.LOL = $'($env.FOO) (lol_shell ls) (ls)'
                 "#,
             )])
             .with_files(vec![FileWithContentToBeTrimmed(
@@ -127,7 +127,7 @@ fn parse_file_relative_to_parsed_file() {
             .with_files(vec![FileWithContentToBeTrimmed(
                 "foo.nu",
                 r#"
-                    let-env FOO = 'foo'
+                    $env.FOO = 'foo'
                 "#,
             )]);
 
@@ -157,13 +157,13 @@ fn parse_file_relative_to_parsed_file_dont_use_cwd_1() {
             .with_files(vec![FileWithContentToBeTrimmed(
                 "lol/foo.nu",
                 r#"
-                    let-env FOO = 'good'
+                    $env.FOO = 'good'
                 "#,
             )])
             .with_files(vec![FileWithContentToBeTrimmed(
                 "foo.nu",
                 r#"
-                    let-env FOO = 'bad'
+                    $env.FOO = 'bad'
                 "#,
             )]);
 
@@ -193,7 +193,7 @@ fn parse_file_relative_to_parsed_file_dont_use_cwd_2() {
             .with_files(vec![FileWithContentToBeTrimmed(
                 "foo.nu",
                 r#"
-                    let-env FOO = 'bad'
+                    $env.FOO = 'bad'
                 "#,
             )]);