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
fmt --all` applies these changes)
- `cargo clippy --workspace -- -D warnings -D clippy::unwrap_used -A
clippy::needless_collect -A clippy::result_large_err` to check that
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-parser/src/parser.rs

@@ -5343,46 +5343,38 @@ pub fn parse_block(
 
                     if idx == 0 {
                         if let Some(let_decl_id) = working_set.find_decl(b"let", &Type::Nothing) {
-                            if let Some(let_env_decl_id) =
-                                working_set.find_decl(b"let-env", &Type::Nothing)
-                            {
-                                for element in pipeline.elements.iter_mut() {
-                                    if let PipelineElement::Expression(
-                                        _,
-                                        Expression {
-                                            expr: Expr::Call(call),
+                            for element in pipeline.elements.iter_mut() {
+                                if let PipelineElement::Expression(
+                                    _,
+                                    Expression {
+                                        expr: Expr::Call(call),
+                                        ..
+                                    },
+                                ) = element
+                                {
+                                    if call.decl_id == let_decl_id {
+                                        // Do an expansion
+                                        if let Some(Expression {
+                                            expr: Expr::Keyword(_, _, expr),
                                             ..
-                                        },
-                                    ) = element
-                                    {
-                                        if call.decl_id == let_decl_id
-                                            || call.decl_id == let_env_decl_id
+                                        }) = call.positional_iter_mut().nth(1)
                                         {
-                                            // Do an expansion
-                                            if let Some(Expression {
-                                                expr: Expr::Keyword(_, _, expr),
-                                                ..
-                                            }) = call.positional_iter_mut().nth(1)
-                                            {
-                                                if expr.has_in_variable(working_set) {
-                                                    *expr = Box::new(wrap_expr_with_collect(
-                                                        working_set,
-                                                        expr,
-                                                    ));
-                                                }
+                                            if expr.has_in_variable(working_set) {
+                                                *expr = Box::new(wrap_expr_with_collect(
+                                                    working_set,
+                                                    expr,
+                                                ));
                                             }
-                                            continue;
-                                        } else if element.has_in_variable(working_set)
-                                            && !is_subexpression
-                                        {
-                                            *element =
-                                                wrap_element_with_collect(working_set, element);
                                         }
+                                        continue;
                                     } else if element.has_in_variable(working_set)
                                         && !is_subexpression
                                     {
                                         *element = wrap_element_with_collect(working_set, element);
                                     }
+                                } else if element.has_in_variable(working_set) && !is_subexpression
+                                {
+                                    *element = wrap_element_with_collect(working_set, element);
                                 }
                             }
                         }