Add _get_unsaved_status() method to EditorPlugin and implement it for script and shader editors

[KoBeWi]

This PR adds _get_unsaved_status(). When editor is being closed, it calls this method on every plugin. The method returns a string. If the string is not empty, its content will be displayed in save confirmation dialog and the user can normally choose to save or discard. If the returned string is empty, the editor will close normally.

I implemented this method for script editor and shader editor, so unsaved scripts will now prompt on editor exit. Unfortunately you can't discard them due to #18010

Fixes #55026
Fixes #36281
Supersedes #55586

[KoBeWi]

Rebased and ready for review. I did a small modification: get_unsaved_status() is now called when closing a scene and passes the scene path as an argument. This allows to handle built-in resources and effectively fixes #44053

godot.windows.editor.dev.x86_64_Ms3syKkV8H.mp4

I added it as another commit, but I can rebase it into 2 commits if that's better.

[YuriSizov]

Haven't tested, but the code looks mostly correct (left a couple of notes).

The feature itself is fine, though I think we should at some point make it more convenient to handle unsaved resources specifically. I proposed before an API that would allow plugins to mark arbitrary strings as dirty, which could be used to identify resources and any other stuff as unsaved.

But maybe we just need this new method and some kind of _get_unsaved_files which returns a list. This can be done in a follow-up though.

[YuriSizov]

Thanks!