deps: use msgpackr

[justinmk]

Evaluate if it's worth switching to https://www.npmjs.com/package/msgpackr instead of our current msgpack dependency.

• See nvim version from env #207 (comment)
• Previous: use a different msgpack lib? #78
• Related: deps: replace msgpack-lite with msgpackr vscode-neovim/vscode-neovim#1316

[gjf7]

It appears that msgpackr does not support using 0 as an extension type code. kriszyp/msgpackr#150

[justinmk]

Thank you for mentioning that. Arg, I really wish we hadn't played with "extension types".