feat(integrations): Optionally autoload keymaps from lazy.nvim plugin specs

[mrjones2014]

Resolves: #377

How to Test

1. Setup legendary.nvim with require('legendary').setup({ lazy_nvim = { auto_register = true } })
2. Set up a keymap through a lazy.nvim plugin spec like

{
  'folke/flash.nvim',
  keys = {
    {
      's',
      function()
        require('flash').jump()
      end,
      mode = { 'n', 'x', 'o' },
      desc = 'Jump forwards',
    },
    {
      'S',
      function()
        require('flash').jump({ search = { forward = false } })
      end,
      mode = { 'n', 'x', 'o' },
      desc = 'Jump backwards',
    },
  },
}

3. The keymaps defined in the plugin spec should be loaded into legendary.nvim

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