Refactored declaration completion

Refactored completion around accessors (public/private/inline/...) and declarations (var/function/class).
This might be missing a few things:

- `to` and `from` in abstracts
- the `import [package] in [name];` (and Haxe 3.2 allows `as` as well: https://github.com/HaxeFoundation/haxe/issues/2361, https://github.com/HaxeFoundation/haxe/commit/3a938dab8e22ea36b81c89cd15c66d9ef1454b7d)