Add commands from nvim_get_commands

[ibash]

This adds commands from nvim_get_commands and attempts to filter out non-human-friendly descriptions.

Contributes https://github.com/mrjones2014/legendary.nvim/issues/259

How to Test

1. Please leave detailed testing instructions

2. Enable built in commands

3. Run :Legendary

4. Look for added commands and check to make sure that non-human-friendly descriptions are not present.

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