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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- `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
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> toolkit check pr
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# 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-command/tests/commands/table.rs

@@ -38,7 +38,7 @@ fn table_collapse_0() {
 #[test]
 fn table_collapse_basic() {
     let actual = nu!(nu_repl_code(&[
-        "let-env config = { table: { mode: basic }};",
+        "$env.config = { table: { mode: basic }};",
         "[[a b, c]; [1 2 3] [4 5 [1 2 3]]] | table --collapse"
     ]));
     assert_eq!(
@@ -60,7 +60,7 @@ fn table_collapse_basic() {
 #[test]
 fn table_collapse_heavy() {
     let actual = nu!(nu_repl_code(&[
-        "let-env config = { table: { mode: heavy }};",
+        "$env.config = { table: { mode: heavy }};",
         "[[a b, c]; [1 2 3] [4 5 [1 2 3]]] | table --collapse"
     ]));
     assert_eq!(
@@ -82,7 +82,7 @@ fn table_collapse_heavy() {
 #[test]
 fn table_collapse_compact() {
     let actual = nu!(nu_repl_code(&[
-        "let-env config = { table: { mode: compact }};",
+        "$env.config = { table: { mode: compact }};",
         "[[a b, c]; [1 2 3] [4 5 [1 2 3]]] | table --collapse"
     ]));
     assert_eq!(
@@ -104,7 +104,7 @@ fn table_collapse_compact() {
 #[test]
 fn table_collapse_compact_double() {
     let actual = nu!(nu_repl_code(&[
-        "let-env config = { table: { mode: compact_double }};",
+        "$env.config = { table: { mode: compact_double }};",
         "[[a b, c]; [1 2 3] [4 5 [1 2 3]]] | table --collapse"
     ]));
     assert_eq!(
@@ -126,7 +126,7 @@ fn table_collapse_compact_double() {
 #[test]
 fn table_collapse_compact_light() {
     let actual = nu!(nu_repl_code(&[
-        "let-env config = { table: { mode: light }};",
+        "$env.config = { table: { mode: light }};",
         "[[a b, c]; [1 2 3] [4 5 [1 2 3]]] | table --collapse"
     ]));
     assert_eq!(
@@ -148,7 +148,7 @@ fn table_collapse_compact_light() {
 #[test]
 fn table_collapse_none() {
     let actual = nu!(nu_repl_code(&[
-        "let-env config = { table: { mode: none }};",
+        "$env.config = { table: { mode: none }};",
         "[[a b, c]; [1 2 3] [4 5 [1 2 3]]] | table --collapse"
     ]));
     assert_eq!(
@@ -166,7 +166,7 @@ fn table_collapse_none() {
 #[test]
 fn table_collapse_compact_reinforced() {
     let actual = nu!(nu_repl_code(&[
-        "let-env config = { table: { mode: reinforced }};",
+        "$env.config = { table: { mode: reinforced }};",
         "[[a b, c]; [1 2 3] [4 5 [1 2 3]]] | table --collapse"
     ]));
     assert_eq!(
@@ -188,7 +188,7 @@ fn table_collapse_compact_reinforced() {
 #[test]
 fn table_collapse_compact_thin() {
     let actual = nu!(nu_repl_code(&[
-        "let-env config = { table: { mode: thin }};",
+        "$env.config = { table: { mode: thin }};",
         "[[a b, c]; [1 2 3] [4 5 [1 2 3]]] | table --collapse"
     ]));
     assert_eq!(
@@ -210,7 +210,7 @@ fn table_collapse_compact_thin() {
 #[test]
 fn table_collapse_hearts() {
     let actual = nu!(nu_repl_code(&[
-        "let-env config = { table: { mode: with_love }};",
+        "$env.config = { table: { mode: with_love }};",
         "[[a b, c]; [1 2 3] [4 5 [1 2 3]]] | table --collapse"
     ]));
     assert_eq!(