Add back 'ui/assets/' to .prettierignore (#58056)

This line was removed in #57365 but it seems it causes `sg lint --fix format` to
format all the generated JS and JSON files inside ui/assets/ and is thus
rather slow. I believe this folder can be safely ignored.

Author: fkling

.prettierignore

@@ -11,6 +11,7 @@ internal/database/migration/shared/data/**/*.json
 cmd/xlang-python/python-langserver/
 package-lock.json
 package.json
+ui/assets/
 src-tauri/assets/
 src-tauri/target/
 vendor/