feat(keys): include custom keys in help menu

[MariaSolOs]

Adds an optional description to custom keys. When present, the key is listed in the help menu.

This is how it looks:

[MariaSolOs]

Not surrounding the keys with angle brackets here because it looked kind of odd with mappings like <localleader>t, but I'm open to other ways to format these.

[folke]

Nice, thanks!!

As a thank you, here are my two custom Telescope keymaps, related to lazy I'm using all the time, that you may find useful as well :)

Especially the seconds one is really useful to quickly navigate to a spec somewhere in your config.

    keys = {
      {
        "<leader>fp",
        function()
          require("telescope.builtin").find_files({
            cwd = require("lazy.core.config").options.root,
          })
        end,
        desc = "Find Plugin File",
      },
      {
        "<leader>fl",
        function()
          local files = {} ---@type table<string, string>
          for _, plugin in pairs(require("lazy.core.config").plugins) do
            repeat
              if plugin._.module then
                local info = vim.loader.find(plugin._.module)[1]
                if info then
                  files[info.modpath] = info.modpath
                end
              end
              plugin = plugin._.super
            until not plugin
          end
          require("telescope.builtin").live_grep({
            default_text = "/",
            search_dirs = vim.tbl_values(files),
          })
        end,
        desc = "Find Lazy Plugin Spec",
      },
    },

[MariaSolOs]

@folke sweet, thanks for the lazy goodies :)