Add autocompletion for OS.has_feature()

[Mickeon]

Closes godotengine/godot-proposals#4240.

The implementation borrows from ProjectSettingsEditor, putting into EditorExport for OS to access.

But this causes a dependency on the editor class...

[Mickeon]

I was answering back but the comment was deleted.
It's like this because it's a copy-paste from ProjectSettingsEditor. There's no real place where these are "stored", quoting Vnen:

This should be easy to add, but I don't think the list of features is available anywhere. So either it should be hardcoded into GDScript auto-completion or we need add a central list of possible features in the editor to harness for the completion list.

Maybe and ideally, the two need to be united somehow.

[AThousandShips]

Indeed, noticed that it was just as I had commented, my bad 🙂

I'd say to either unite it with the other one, or use the list directly to avoid the step

[Mickeon]

I think I came with a nice compromise, but the OS needs to know about EditorExport which is still not nice, so perhaps as a last resort I would have to hardcode it like #76591 has done, into gdscript_editor.cpp. I don't find it to be neatly organized, but it would not be... dependency-d... dependent?

[Mickeon]

Thank you. It'll probably compile now, but we'll then see what are the thoughts on code quality