Add missing comma in string list leading to implicit concat

In python string list literals, if you you accidentally miss a comma,
the two values get concatenated due to Python implicit string concatenation.
This can sometimes lead to errors that are very hard to find and debug.

This was found using a regex and then manually verified to be a bug.

Change-Id: I8f58b45340d1f33808dcd892037fe64dee5aedff
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3107685
Reviewed-by: Ken Rockot <rockot@google.com>
Commit-Queue: Ehsan Kia <ehsankia@google.com>
Cr-Commit-Position: refs/heads/main@{#913843}

mojo/public/tools/bindings/generators/mojom_js_generator.py

@@ -130,7 +130,7 @@ _kind_to_lite_js_type = {
 _js_reserved_keywords = [
     'arguments',
     'await',
-    'break'
+    'break',
     'case',
     'catch',
     'class',