Add support for Godot 4 annotations

[vertexmachina]

This enables the highlighting of annotations in the form of @export, @export_multiline, @onready, etc.

I'm new to writing Vim syntax highlighting files so it's possible I did this in a non-idiomatic way. If there's a more concise way of matching all lowercase and underscores following an @, please let me know.

Perhaps ^@[a-z_]\+?