[Feat] Add JSON output to r.what

[petrasovaa]

Add JSON output for easy parsing of r.what output.

Describe the solution you'd like
Adding option format, it would be something like format=plain for the current output format and format=json for the JSON output. There is a newly added parson library to GRASS (#3028), so it should be used here.

See also discussion in #3019.

A test should be included.

[kritibirda26]

Thanks for opening the issue, I'll start looking into this,

[kritibirda26]

Hi! I had some trouble working with parson because there is no other use in the codebase so far but I was eventually able to figure it out. Hence, I have opened a PR above to add the JSON support. Kindly review whenever possible.