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/tests/commands/break_.rs

@@ -0,0 +1,37 @@
+use nu_test_support::{nu, pipeline};
+
+#[test]
+fn break_for_loop() {
+    let actual = nu!(
+        cwd: ".", pipeline(
+        r#"
+        for i in 1..10 { if $i == 2 { break }; print $i }
+        "#
+    ));
+
+    assert_eq!(actual.out, r#"1"#);
+}
+
+#[test]
+fn break_while_loop() {
+    let actual = nu!(
+        cwd: ".", pipeline(
+        r#"
+        while true { break }; print "hello"
+        "#
+    ));
+
+    assert_eq!(actual.out, r#"hello"#);
+}
+
+#[test]
+fn break_each() {
+    let actual = nu!(
+        cwd: ".", pipeline(
+        r#"
+        [1, 2, 3, 4, 5] | each {|x| if $x > 3 { break }; $x} | math sum
+        "#
+    ));
+
+    assert_eq!(actual.out, r#"6"#);
+}

crates/nu-command/tests/commands/continue_.rs

@@ -0,0 +1,13 @@
+use nu_test_support::{nu, pipeline};
+
+#[test]
+fn continue_for_loop() {
+    let actual = nu!(
+        cwd: ".", pipeline(
+        r#"
+        for i in 1..10 { if $i == 2 { continue }; print $i }
+        "#
+    ));
+
+    assert_eq!(actual.out, r#"1345678910"#);
+}

crates/nu-command/tests/commands/mod.rs

@@ -2,9 +2,11 @@ mod alias;
 mod all;
 mod any;
 mod append;
+mod break_;
 mod cal;
 mod cd;
 mod compact;
+mod continue_;
 mod cp;
 mod date;
 mod def;
@@ -61,6 +63,7 @@ mod redirection;
 mod reduce;
 mod reject;
 mod rename;
+mod return_;
 mod reverse;
 mod rm;
 mod roll;

crates/nu-command/tests/commands/return_.rs

@@ -0,0 +1,25 @@
+use nu_test_support::{nu, pipeline};
+
+#[test]
+fn early_return_if_true() {
+    let actual = nu!(
+        cwd: ".", pipeline(
+        r#"
+        def foo [x] { if true { return 2 }; $x }; foo 100
+        "#
+    ));
+
+    assert_eq!(actual.out, r#"2"#);
+}
+
+#[test]
+fn early_return_if_false() {
+    let actual = nu!(
+        cwd: ".", pipeline(
+        r#"
+        def foo [x] { if false { return 2 }; $x }; foo 100
+        "#
+    ));
+
+    assert_eq!(actual.out, r#"100"#);
+}