I would recommend to ship a `JSON with comments` filetype from the start because it does seem to be somewhat common to me (mostly due to MS's VS Code which popularized). Being able to extend it is always cool, but obviously the user experience is better if the right file mode is already available in the built-in list. With that given, it seems reasonable to then make the "regular" `JSON` type be strict about comments being not allowed and all that
Fwiw, "JSON with comments" usually means allowing trailing commas in objects and lists, comment type `// line comment`, comment type `/* block comment */`, and sometimes comment type `# alternate line comment`. I would recommend accepting these all as valid in a `JSON with comments` file type if you choose to provide one