Unignore Cargo.lock file

This makes it much easier to checkout a previous revision of the project and have a working set of dependencies. That helps for example with things like running `git bisect` when finding regressions.

see also https://blog.rust-lang.org/2023/08/29/committing-lockfiles.html

.gitignore

@@ -1,4 +1,3 @@
 target
-Cargo.lock
 .DS_Store
 .vscode