[gitignore] add rust-project.json

Copying this from fuchsia.git

Change-Id: I9fc2347ea53f2d8b32f5bb70122b9b0706006d75
Reviewed-on: https://fuchsia-review.googlesource.com/c/experiences/+/397579
Reviewed-by: Jason Campbell <jasoncampbell@google.com>
Testability-Review: Chase Latta <chaselatta@google.com>
Commit-Queue: Chase Latta <chaselatta@google.com>
diff --git a/.gitignore b/.gitignore
index acf45aa..8c82f43 100644
--- a/.gitignore
+++ b/.gitignore
@@ -30,3 +30,5 @@
 /local/
 /tmp/
 
+# rust-analyzer
+/rust-project.json