Add icons to Cmd Palette actions

[tyler-dane]

Context
Command palette support was added in #14 #102.

There are a few minor shortcomings with it, though:

1. The tooltip text says, "Open command palette (CMD+k)".
   Although the text says 'CMD', the code actually uses [Meta Key] + k, where Meta Key is CMD on mac or WINDOWS KEY on windows. Without knowing that, however, Windows users might suspect that the command palette only works for mac users.
2. The cmd palette wasn't tested on Windows
   I'm on Mac and have never tested that the command palette works on Windows.

Tasks

• Update tooltip text so that it shows "{Cmd} + k" for mac users and "{Win} + k" for windows users*, where {Cmd} and {Win} refer to the icons

If you're a Windows user:

• Test it works on Windows and post a comment here to confirm

Implementation Guidance
Update MetaKey.tsx
See icon prop in the react-cmdk ListItem API

[tyler-dane]

See the 'custom' section below for an example of how to use navigator.platform to accomplish this

https://github.com/albingroen/react-cmdk