Disallow usage of ignored `_` locals

[Gama11]

I'm proposing that the following code should throw "ignored variables cannot be used" errors.

class Main {
	static function main() {
		for (_ in 0...16) {
			trace(_); // error
		}
	}

	static function foo(_) {
		trace(_); // error
	}
}

I think it's a weird situation that _ has special semantics in pattern matching, bind(), and now even diagnostics mode for locals (#8147), but that you can still use them as if they were regular vars. The common convention for this is already that they're "ignored", so actually using a _ var would be considered bad style anyway. If an error is introduced, _ can be properly documented as a laguage feature for locals.

I know that at least Flambe uses _ as an identifer, but that's only on field level I think? So shouldn't be impacted by this.

This might also allow to revisit #4381.

[terurou]

I hope this proposal will be approved. (related #8076)

[Gama11]

@terurou Thanks, I had forgotten about that issue actually. So yeah, if the compiler was a bit stricter here, it could definitely help avoid some confusion.