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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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-protocol/tests/into_config.rs

@@ -2,7 +2,7 @@ use nu_test_support::{nu, nu_repl_code};
 
 #[test]
 fn config_is_mutable() {
-    let actual = nu!(cwd: ".", nu_repl_code(&[r"let-env config = { ls: { clickable_links: true } }",
+    let actual = nu!(cwd: ".", nu_repl_code(&[r"$env.config = { ls: { clickable_links: true } }",
         "$env.config.ls.clickable_links = false;",
         "$env.config.ls.clickable_links"]));
 
@@ -11,7 +11,7 @@ fn config_is_mutable() {
 
 #[test]
 fn config_preserved_after_do() {
-    let actual = nu!(cwd: ".", nu_repl_code(&[r"let-env config = { ls: { clickable_links: true } }",
+    let actual = nu!(cwd: ".", nu_repl_code(&[r"$env.config = { ls: { clickable_links: true } }",
         "do -i { $env.config.ls.clickable_links = false }",
         "$env.config.ls.clickable_links"]));
 
@@ -20,7 +20,7 @@ fn config_preserved_after_do() {
 
 #[test]
 fn config_affected_when_mutated() {
-    let actual = nu!(cwd: ".", nu_repl_code(&[r#"let-env config = { filesize: { metric: false, format:"auto" } }"#,
+    let actual = nu!(cwd: ".", nu_repl_code(&[r#"$env.config = { filesize: { metric: false, format:"auto" } }"#,
         r#"$env.config = { filesize: { metric: true, format:"auto" } }"#,
         "20mib | into string"]));
 

crates/nu-protocol/tests/test_config.rs

@@ -3,7 +3,7 @@ use nu_test_support::{nu, nu_repl_code};
 #[test]
 fn filesize_metric_true() {
     let code = &[
-        r#"let-env config = { filesize: { metric: true, format:"mb" } }"#,
+        r#"$env.config = { filesize: { metric: true, format:"mb" } }"#,
         r#"20mib | into string"#,
     ];
     let actual = nu!(cwd: ".", nu_repl_code( code ));
@@ -13,7 +13,7 @@ fn filesize_metric_true() {
 #[test]
 fn filesize_metric_false() {
     let code = &[
-        r#"let-env config = { filesize: { metric: false, format:"mib" } }"#,
+        r#"$env.config = { filesize: { metric: false, format:"mib" } }"#,
         r#"20mib | into string"#,
     ];
     let actual = nu!(cwd: ".", nu_repl_code( code ));
@@ -23,7 +23,7 @@ fn filesize_metric_false() {
 #[test]
 fn filesize_metric_overrides_format() {
     let code = &[
-        r#"let-env config = { filesize: { metric: false, format:"mb" } }"#,
+        r#"$env.config = { filesize: { metric: false, format:"mb" } }"#,
         r#"20mib | into string"#,
     ];
     let actual = nu!(cwd: ".", nu_repl_code( code ));
@@ -33,7 +33,7 @@ fn filesize_metric_overrides_format() {
 #[test]
 fn filesize_format_auto_metric_true() {
     let code = &[
-        r#"let-env config = { filesize: { metric: true, format:"auto" } }"#,
+        r#"$env.config = { filesize: { metric: true, format:"auto" } }"#,
         r#"[2mb 2gb 2tb] | into string | to nuon"#,
     ];
     let actual = nu!(cwd: ".", nu_repl_code( code ));
@@ -43,7 +43,7 @@ fn filesize_format_auto_metric_true() {
 #[test]
 fn filesize_format_auto_metric_false() {
     let code = &[
-        r#"let-env config = { filesize: { metric: false, format:"auto" } }"#,
+        r#"$env.config = { filesize: { metric: false, format:"auto" } }"#,
         r#"[2mb 2gb 2tb] | into string | to nuon"#,
     ];
     let actual = nu!(cwd: ".", nu_repl_code( code ));