feat(extension)!: Move which-key.nvim and lazy.nvim integrations to extensions

[mrjones2014]

Resolves: #399

How to Test

1. Use both the integrations
2. Launching nvim with existing config should trigger deprecation warnings for both their settings, saying they've moved to extensions
3. Enabling the extensions, everything should still work exactly the way it did before

Testing for Regressions

I have tested the following:

• Triggering keymaps from legendary.nvim in all modes (normal, insert, visual)
• Creating keymaps via legendary.nvim, then triggering via the keymap in all modes (normal, insert, visual)
• Triggering commands from legendary.nvim in all modes (normal, insert, visual)
• Creating commands via legendary.nvim, then running the command manually from the command line
• augroup/autocmds created through legendary.nvim work correctly