json.lang: errors for trailing commas
Currently when editing a JSON flatpak manifest (or any JSON file), it can be difficult to notice where a trailing comma was left that prevented it from working, especially since most programming languages allow (or even enforce) them. The json.lang already has a bit of error styling when e.g. inserting a non allowed character at this position (like a something else than a comma after a value), but it doesn't show the error styling for trailing commas, which is probably the most useful case for an error styling here.
The json.lang file indicates that "If once-only or end-parent is ever supported for context-reference and context-to-be-included elements"
then trailing comma error styling could be implemented. Is there support for such context attributes now, or is there any other way to implement trailing comma support without them?