New commands: break, continue, return, and loop (#7230)

# Description

This adds `break`, `continue`, `return`, and `loop`.

* `break` - breaks out a loop
* `continue` - continues a loop at the next iteration
* `return` - early return from a function call
* `loop` - loop forever (until the loop hits a break)

Examples:
```
for i in 1..10 {
    if $i == 5 {
       continue
    } 
    print $i
}
```

```
for i in 1..10 {
    if $i == 5 {
        break
    } 
    print $i
}
```

```
def foo [x] {
    if true {
        return 2
    }
    $x
}
foo 100
```

```
loop { print "hello, forever" }
```

```
[1, 2, 3, 4, 5] | each {|x| 
    if $x > 3 { break }
    $x
}
```

# User-Facing Changes

Adds the above commands.

# 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` to check that you're using the standard code
style
- `cargo test --workspace` to check that all tests pass

# After Submitting

If your PR had any user-facing changes, update [the
documentation](https://github.com/nushell/nushell.github.io) after the
PR is merged, if necessary. This will help us keep the docs up to date.

crates/nu-command/src/example_test.rs

@@ -12,7 +12,7 @@ mod test_examples {
         Ansi, Date, Echo, From, If, Into, LetEnv, Math, Path, Random, Split, SplitColumn, SplitRow,
         Str, StrJoin, StrLength, StrReplace, Url, Wrap,
     };
-    use crate::To;
+    use crate::{Break, Mut, To};
     use itertools::Itertools;
     use nu_protocol::{
         ast::Block,
@@ -80,6 +80,8 @@ mod test_examples {
             working_set.add_decl(Box::new(Wrap));
             working_set.add_decl(Box::new(LetEnv));
             working_set.add_decl(Box::new(Echo));
+            working_set.add_decl(Box::new(Break));
+            working_set.add_decl(Box::new(Mut));
             // Adding the command that is being tested to the working set
             working_set.add_decl(cmd);