[null-safety] Check structures and better errors

[RblSb]

Found bug that {?a:Int} still can be passed to {a:Int} var.

Also added better errors usage, so instead of

Cannot unify { ?a : Null<Int> } with haxeLanguageServer.documents.OnTextDocumentChangeListener2

There's now:

Null safety: error: Null<Int> should be Int
have: { a: Null<Int> }
want: { a: Int }

Closes #7855
Closes #12454