ignore vendor directory in git status

Author: jyn514

.gitignore

@@ -54,6 +54,8 @@ no_llvm_build
 /library/target
 /src/bootstrap/target
 /src/tools/x/target
+# Created by `x vendor`
+/vendor
 # Created by default with `src/ci/docker/run.sh`
 /obj/
 # Created by nix dev shell / .envrc