Add keymap description option

[nat-418]

nvim-treesitter-refactor keymaps cannot be described as in nvim-treesitter-textobjects or using vim.keymap.set mappings.

It would be nice if nvim-treesitter-refactor supported the same structure as nvim-treesitter-textobjects, e.g.,

keymaps = {
  [']u'] = {
    action = goto_next_usage,
    desc = 'Next usage'
  }
}

Implementing this feature will provide better integration with the Neovim ecosystem of plugins like which-key and legendary.