Refactor source view states to use base class and fix open-files start path

- Add SourceViewsBaseState inheritance to Command, Insert, MultiInsert, and ReadOnly states
- Extract common focus_in_event logic to base class
- Fix open-files command to use current file's directory as file picker start path
- Extract modifier key state logic into reusable get_modkeys_states() method
- Pass modifier keys to command exec_with_args in key_press_event
- Add type hints to key_mapper methods

plugins/code/autopairs/__main__.py

@@ -0,0 +1,3 @@
+"""
+    Plugin Package
+"""