refactor(command-system): standardize command execution with *args/**kwargs

- Refactor exec_with_args to use *args/**kwargs instead of tuple arguments
- Add *args/**kwargs to all command execute functions for consistency
- Support multiple key bindings per command in registration
- Add character-based key binding support via get_char() in KeyMapper
- Make execute_plugin async and use asyncio.run for plugin execution
- Use MIME type from Gio content_type instead of language for ftype

user_config/usr/share/newton/stylesheet.css

@@ -111,6 +111,7 @@ scrollbar slider:active {
 .tab-label {
     margin-left: 2em;
     margin-right: 2em;
+    font-size: 12px;
 }
 
 .tab-close-bttn {