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-protocol/src/shell_error.rs

@@ -2,7 +2,7 @@ use miette::Diagnostic;
 use serde::{Deserialize, Serialize};
 use thiserror::Error;
 
-use crate::{ast::Operator, Span, Type};
+use crate::{ast::Operator, Span, Type, Value};
 
 /// The fundamental error type for the evaluation engine. These cases represent different kinds of errors
 /// the evaluator might face, along with helpful spans to label. An error renderer will take this error value
@@ -828,6 +828,18 @@ Either make sure {0} is a string, or add a 'to_string' entry for it in ENV_CONVE
     #[error("Eval block failed with pipeline input")]
     #[diagnostic(code(nu::shell::eval_block_with_input), url(docsrs))]
     EvalBlockWithInput(#[label("source value")] Span, #[related] Vec<ShellError>),
+
+    /// Break event, which may become an error if used outside of a loop
+    #[error("Break used outside of loop")]
+    Break(#[label = "used outside of loop"] Span),
+
+    /// Continue event, which may become an error if used outside of a loop
+    #[error("Continue used outside of loop")]
+    Continue(#[label = "used outside of loop"] Span),
+
+    /// Return event, which may become an error if used outside of a function
+    #[error("Return used outside of function")]
+    Return(#[label = "used outside of function"] Span, Box<Value>),
 }
 
 impl From<std::io::Error> for ShellError {