Remove nextpack (#4266)
Nextpack had various problems:
- It failed VSCode if nextpack wasn't initialized
- The dir hierarchy didn't play well with Vim's COC
- The `.gitignore` prevented ripgrep from finding files (both grepping
and fuzzy file path opening)
We're instead switching to the internal nextpack repo for development on
both the next.js and turbo repos.
Closes https://github.com/vercel/turbo/issues/4260
