- Implements the concept of GDExtension loaders that can be used to customize how GDExtensions are loaded and initialized.
- Moves the parsing of `.gdextension` config files to the new `GDExtensionLibraryLoader`.
- `GDExtensionManager` is now meant to be the main way to load/unload extensions and can optionally take a `GDExtensionLoader`.
- `EditorFileSystem` avoids unloading extensions if the file still exists, this should prevent unloading extensions that are outside the user project.
