build: Update prettierrc to always use trailing commas (angular#58266)

This updates the prettierrc to reflect how the code seems to already be
formatted. I noticed that without this my IDE seemed to be introducing
formatting changes on save.

PR Close angular#58266

Author: mmalerba

.prettierrc

@@ -6,5 +6,6 @@
   "singleQuote": true,
   "semi": true,
   "quoteProps": "preserve",
-  "bracketSpacing": false
+  "bracketSpacing": false,
+  "trailingComma": "all"
 }