[chore] Use git for file source

[brendan-kellam]

Currently, we are using zoekt to power our file source api (/api/source). We should switch to using git show <hash>:<path> instead. Main benefits 1) we can get the file source of files on non-indexed branches at arbitrary commit SHAs, and 2) we can get the file source of binary files (like PDFs).

See packages/web/features/search/fileSourceApi.ts