chore: fix unmutable environment

[michaeljklein]

Description

Problem*

Resolves #4795

Summary*

This PR simply adds a check on mutable captures: when not &mut, a type checking error is thrown.

Additional Context

Documentation*

Check one:

• No documentation needed.
• Documentation included in this PR.
• [For Experimental Features] Documentation to be submitted in a separate PR.

PR Checklist*

• I have tested the changes locally.
• I have formatted the changes with Prettier and/or cargo fmt on default settings.

[jfecher]

Instead of adding a type check on a lambda's environment, can we avoid making the captured variables mutable in the first place? When the lookup is performed in name resolution there should be a check to look up a variable from an outer function. We should ensure there that the variable is introduced only as immutable if possible.

[jfecher]

Suggested change

	let captured_vars =
	match self.ensure_mutable_captures_have_ref(expr_id, lambda.captures) {
	Err(err) => {
	self.errors.push(err);
	return Type::Error;
	}
	Ok(captured_vars) => captured_vars,
	};
	let Ok(captured_vars) = self.ensure_mutable_captures_have_ref(expr_id, lambda.captures) else {
	self.errors.push(err);
	return Type::Error;
	};