Turn the native messenger into a neovim plugin

[glacambre]

This should be fairly easy to do:

• Have a :FirenvimInstall command that creates a {power,}shell script in ~/.local/share/firenvim and puts the native manifest in ~/.mozilla/native-messaging-hosts
• On launch, have the neovim plugin open a tcp port (Feature Request: allow opening ports from lua neovim/neovim#10084) and generate a security token, then send token+port number to firefox with stdioopen()
• Have the webextension open a websocket, send the token to the neovim plugin and then use regular msgpack-rpc messages until end of connection

This is probably going to make starting a new firenvim frame rather slow - we might want to instantiate one neovim process in the background in order to speed things up.

[glacambre]

This seems to work on windows and linux. Just need to wait for nvim 4.0 to become stable before releasing an updated version on amo.
https://github.com/neovim/neovim/milestone/21