gitignore root dir cargo builds

Summary: open-source rust-analyzer seems to build the root `Cargo.toml`. it generates a `targets/` dir in pyrefly root along with `Cargo.lock`. I looked into ways to get it to only analyze the child `Cargo.toml` with no luck. Ignoring these is a much better user-experience.

Reviewed By: ndmitchell

Differential Revision: D75459926

fbshipit-source-id: c9ff3aa4f059d142f3b7860779c8285407bc4557

Author: kinto0

.gitignore

@@ -1,2 +1,4 @@
 debug.json
 debug.js
+Cargo.lock
+target/