Add .jsonc extension

[nathanshelly]

Marks .jsonc files as jsonc.

Checklist:

• I am associating a language with a new file extension.
  • The new extension is used in hundreds of repositories on GitHub.com
    • Search results for each extension:
      • https://github.com/search?p=1&q=extension%3Ajsonc+NOT+dkfgjdkfjd&type=Code
  • I have included a real-world usage sample for all extensions added in this PR:
    • Sample source(s): https://github.com/neoclide/coc.nvim
    • Sample license(s): MIT

[nathanshelly]

This PR adds the .jsonc extension that was in the initial draft of #4171. It was removed per this comment. I'm adding it back hoping that usage is high enough to accept it this time. Feel free to close out immediately if it isn't.

[Alhadis]

Usage checks out this time.

• Files: 2,557 (out of ~4,250)
• Repos: 648
• Users: 558

[nathanshelly]

Could anyone explain how I would get GitHub to highlight this .jsonc file as jsonc? That was my intent with opening this PR, apologies for apparently misunderstanding how GitHub's highlighting works.

I see this section about issues with syntax highlighting but as far as I can tell the file isn't being recognized as a jsonc filetype so the issue happens before the upstream grammar is even applied.

[Alhadis]

@nathanshelly Changes to Linguist don't become visible on GitHub until there's a new release, which @lildude is in the process of preparing. You'll see the highlighting once #4816 goes live.

[nathanshelly]

🤦‍♂️ of course that makes perfect sense, thanks!