Make timeit take only closures as an argument (#14483)

# Description

Fixes #14401 where expressions passed to `timeit` will execute twice.
This PR removes the expression support for `timeit`, as this behavior is
almost exclusive to `timeit` and can hinder migration to the IR
evaluator in the future. Additionally, `timeit` used to be able to take
a `block` as an argument. Blocks should probably only be allowed for
parser keywords, so this PR changes `timeit` to instead only take
closures as an argument. This also fixes an issue where environment
updates inside the `timeit` block would affect the parent scope and all
commands later in the pipeline.

```nu
> timeit { $env.FOO = 'bar' }; print $env.FOO
bar
```

# User-Facing Changes

`timeit` now only takes a closure as the first argument.

# After Submitting

Update examples in the book/docs if necessary.

crates/nu-command/tests/commands/debug/timeit.rs

@@ -2,7 +2,7 @@ use nu_test_support::nu;
 
 #[test]
 fn timeit_show_stdout() {
-    let actual = nu!("let t = timeit nu --testbin cococo abcdefg");
+    let actual = nu!("let t = timeit { nu --testbin cococo abcdefg }");
     assert_eq!(actual.out, "abcdefg");
 }