GUI, Manual Mode, Offhand

Finally a new version! Major code refactoring, optimizations and new functionality!

• Add offhand config
• Add manual mode
• Add GUI and a keybind
• Compatibility with OffersHUD
• Drop 1.16.x support completely (goodbye!)
• Version support is now from 1.17 up to 1.21.1 inclusive

Technical changes:

• Fix undetected issue with Messages.sendError(context.getSource(), "librgetter.find_librarian");
• Change order of checking for profession loss
• Split multiversion utils in groups.
• Provide abstraction for Plugin parsing (easier to add support for new plugins)
• Bump java version to 17

Fixes #26, #27 and #28