feat(api): Add API to start finder from a specified root item group

[mrjones2014]

Resolves: #390

How to Test

1. Run require('legendary').find({ itemgroup = 'Name of an item group' })
2. It should start the finder with just the items inside that item group

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