[Tab completion] final cleanup

[vlada-shubina]

• ensure legacy code is removed (CommandParsing folder)
• ensure no TODOs - all remaining TODOs are identified and extracted to tasks.
• ensure all relevant strings are localized and not used ones are removed
  • I've made a pass to check if all the required strings are localized and fixed remaining in [system-command-line] some cleanup fixes #4238
• ensure file per class everywhere - [feature/system-command-line] refactoring #4249
• ensure legacy functionality is split to separate file as partial class
• document main classes when missing