feat: Add JSON plugin

[nzakas]

This PR sets up the baseline for the JSON plugin and this repo.

Some notes:

• I purposely kept the implementation simple and only implemented the things necessary to create the first couple of rules. This doesn't preclude adding more functionality to JSONSourceCode later on.
• I also purposely kept the documentation light. We can always add more as we go.
• This currently supports both JSON and JSONC. I'll spend some time looking at JSON5 later on.
• The two rules were the only ones I could think of off the top of my head. We can definitely add more later on. These were the simplest proofs-of-concept that I could come up with.

[mdjermanovic]

I think the whole tests/fixtures folder can be removed.

[mdjermanovic]

LGTM, thanks!