[Editor] Add .editorconfig to the projects.

[bruvzg]

Adds .editorconfig to ensure external editors/IDEs use UTF-8 encoding.

Fixes #27083
Supersede #96837

[aaronfranke]

Note that this will add a .editorconfig into existing projects. In the event that this is desired only for new projects, this code could be moved to editor/project_manager/project_dialog.cpp inside ProjectDialog::ok_pressed(). However, I am not against retroactively adding this file personally, I'm all for it 👍

[bruvzg]

Note that this will add a .editorconfig into existing projects.

It this case, it's better to add it to the existing projects, Godot always used exclusively UTF-8, so it should not be an issue. And you can edit the file if you want different config (it won't overwrite existing one).

[akien-mga]

Thanks!

[AThousandShips]

This injects .editorconfig in modules/gdscript/tests/scripts/ when running tests but it isn't ignored