The JSON language server looks for a top-level `allowTrailingCommas`
flag to decide whether it should warn for trailing commas. Since the
JSONC parser for these builtin files can handles trailing commas, adding
this flag to the schema also prevents a warning for those commas.

I don't think there's an issue that is only for this specific issue, but
it relates to *many* existing / older issues:
- #18509
- #17487
- #40970
- #18509
- #21303

Release Notes:

- Suppress warning for trailing commas in builtin JSON files
(`settings.json`, `keymap.json`, etc.)