Add `.json` extension for `application/jf2feed+json`

[binyamin]

If this is good to go, I can open a PR https://github.com/jshttp/mime-db/blob/c9242a9b7d4bb25d7a0c9244adec74aeef08d8a1/src/iana-types.json#L1028-L1032

[binyamin]

Note that .jf2 is used as well, although the IANA page doesn't mention it.

[dougwilson]

Hello, and thank you for the issue. I'm not sure what the implications would be for consumers of this module that attempt to build a reverse lookup (extension to type) would be. Investigation would need to be performed.