chore(.gitignore): ignore lock files (#8368)
chore(.gitignore): ignore lock files (#8368)

Two reasons:
1. Sometimes these accidentally make it into PRs  (e.g. #8344)
2. Some editor plugins (like the git in vscode) update very frequently causing these files to appear and disappear quickly in the sidebar whenever lean compiles which is annoying





leanprover-community/mathlib,leanprover-community/mathlib