added application/json plugin for sparql results parser

[daghovland]

Summary of changes

Fixes issue #2887

This just adds the mediatype "application/json" to the list of mediatypes that are handled by JSONResultParser. Previously only "application/sparql-results+json" and" json" were handled.

This does not change any existing functionality, only allows one more mediatype to be parsed in the same way as json is parsed. I dont think there can be any negative impact of assuming sparql answers of type application/json are applicaiton/sparql-results+json, as i have not heard of any other json sparql result formats. It would also follow the same pattern as used for application/sparql-results+xml

Checklist

• [x] Checked that there aren't other open pull requests for the same change.
• [x] Checked that all tests and type checking passes.
• If the change adds new features or changes the RDFLib public API:
  • [x] Created an issue to discuss the change and get in-principle agreement.
  • [x] Considered adding an example in ./examples.
• [x] Considered granting push permissions to the PR branch, so maintainers can fix minor issues and keep your PR up to date.

[daghovland]

Incorrect fix. See issue comment.