[nullsafety] Non-nullable check warnings

[RblSb]

Cannot undestand problem with warning diagnostics, need some help:
(nullsafety warnings will stay if there is nullsafety errors btw)

@:nullSafety
@:haxe.warning("-WRedundantNullCheck")
class Main {
	static function main() {
		new Main();
	}

	function new() {
		final foo = "1";
		// this warning doesn't react on WRedundantNullCheck
		// and also hides after vscode tab swap
		final v = foo ?? "0";

		final foo = 1;
		// no warning because of Null<Int> cast in operator
		// probably can be fixed with some @:nonNullable internal meta on expr
		final v = foo ?? 0;

		// working warning example
		final f:Int;
		function name():Void {
			f + 1;
		}
	}
}

Closes #7816

[kLabz]

I will try to fix diagnostics vs cache early next week, that behavior is currently more or less expected.

Edit: was mostly about haxe.warning rules vs cached modules, but there are several things wrong in this PR that I'm working on fixing

[kLabz]

Reworked to make it work with type level @:haxe.warning rules:
kLabz@d9939f9

[Kinda] works with:

@:nullSafety
@:haxe.warning("+WRedundantNullCheck") // Notice the "+" instead of "-"
class Main {
	static function main() {}

	// TODO
	// @:haxe.warning("+WRedundantNullCheck")
	function new() {
		final foo = "1";
		// this warning does react to +WRedundantNullCheck
		// but still hides after vscode tab swap (warnings added in filter happen too late.. need to think about it)
		final v = foo ?? "0";
		ignore(v);
	}

	static function ignore<T>(v:T):Void {}
}

Field/expression level is doable but more annoying to handle. Can definitely be done, though.
Will look into that warnings in filters vs cache thing