Update gitignore for yarn files (#316)

Author: kevinwcyu

.gitignore

@@ -12,3 +12,12 @@ yarn-error.log
 .DS_Store
 
 .eslintcache
+
+# yarn
+.pnp.*
+.yarn/*
+!.yarn/patches
+!.yarn/plugins
+!.yarn/releases
+!.yarn/sdks
+!.yarn/versions