Optimize SceneTree's change_scene_to_file autocompletion

[Mickeon]

As is currently, the autocompletion for the SceneTree's change_scene_to_file is handled within get_argument_options. The implementation is done using a DirAccess and iterating through every file recursively, every time. It turns out this has a few problems:

• It is EXTREMELY slow, especially on larger projects.
  • At least on my machine, I have definitely seen it struggle.
• It does not account for the way files are imported.
  • Only file extensions are checked. This is not enough because a PackedScene could be imported in other ways, as well (such as .glb files).
• It does not respect the user's "text_editor/completion/complete_file_paths" Editor Setting.
• It's not scalable. This is not the only method that would benefit from file path autocompletion

This PR moves the autocompletion code out of the SceneTree and into the autocompletion code itself, recycling an existing function to go around all imported PackedScenes. This uses of the editor's own EditorFileSystem and is overall more performant.

In the future, this code snippet can even be further expanded with other similar methods.

[KoBeWi]

Tested in my project with 1000+ scenes and this makes a nice speedup.

[akien-mga]

Needs a rebase.

[Mickeon]

All good now

[Mickeon]

All gooder now.

[akien-mga]

Thanks!