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
<!--
Don't forget to add tests that cover your changes.

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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- `cargo clippy --workspace -- -D warnings -D clippy::unwrap_used -A
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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)

crates/nu-std/std/dirs.nu

@@ -17,8 +17,8 @@
 # This situation could arise if we started with [/a, /b, /c], then
 # we changed directories from /b to /var/tmp.
 export-env {
-    let-env DIRS_POSITION = 0
-    let-env DIRS_LIST = [($env.PWD | path expand)]
+    $env.DIRS_POSITION = 0
+    $env.DIRS_LIST = [($env.PWD | path expand)]
 }
 
 # Add one or more directories to the list.
@@ -36,7 +36,7 @@ export def-env add [
             $abspaths = ($abspaths | append $exp)
         }
 
-        let-env DIRS_LIST = ($env.DIRS_LIST | insert ($env.DIRS_POSITION + 1) $abspaths | flatten)
+        $env.DIRS_LIST = ($env.DIRS_LIST | insert ($env.DIRS_POSITION + 1) $abspaths | flatten)
 
 
     _fetch 1
@@ -66,7 +66,7 @@ export alias p = prev
 # PWD becomes the next working directory
 export def-env drop [] {
     if ($env.DIRS_LIST | length) > 1 {
-        let-env DIRS_LIST = ($env.DIRS_LIST | reject $env.DIRS_POSITION)
+        $env.DIRS_LIST = ($env.DIRS_LIST | reject $env.DIRS_POSITION)
         if ($env.DIRS_POSITION >= ($env.DIRS_LIST | length)) {$env.DIRS_POSITION = 0}
     }
 
@@ -111,7 +111,7 @@ export def-env goto [shell?: int] {
             }
         }
     }
-    let-env DIRS_POSITION = $shell
+    $env.DIRS_POSITION = $shell
 
     cd ($env.DIRS_LIST | get $env.DIRS_POSITION)
 }