GDScript: Add autocompletion for @export_tool_button

[dalexeev]

[HolonProduction]

I'd usually say we should quote all argument option preview strings for consistency, but in this case it makes sense to not do that, since the editor won't show the icons for quoted names.

With this PR the autocompletion depends on EditorNode, thus the completion test case should be marked as editor test case. Also a basic test case for this would be nice (if the theme is available in tests that is).

[Mickeon]

In my opinion this should've been done sooner than later. The icon parameter may as well not exist because most users don't realise what is available to them or (reasonably) assume it works like @icon.

[Repiteo]

Thanks!