fix(api): Apply itemgroup filter correctly

[mrjones2014]

Resolves: #435

How to Test

1. Have an item group
2. Trigger the API via require('legendary').find({ itemgroup = "my group" })
3. It should start the finder with items from the 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