feat: add popup_user_mappings option

[younger-1]

Summary

This PR add a WK option popup_user_mappings (table: string -> function).

With this option, user can binding key to function inside the WK popup.

The user-defined function accepts two parameter (keys, mode) , in which keys is the keys bring up WK popup, mode is the current vim mode.

Use case

my use case is to fuzzy find the keymaps with above keys as a prefix.
For example to mimic emacs' <C-h>, with Telescope keymaps :

  popup_user_mappings = {
    ['<C-h>'] = function(key, mode)
      require('telescope.builtin').keymaps {
        modes = { mode },
        lhs_filter = function(lhs)
          return vim.startswith(lhs, key)
        end,
      }
    end,
  },