Add Reviewdog Diagnostic Format

[wsipak]

This PR adds a new output format that can be selected using --output_format flag of the linter.
The format is described here:
https://github.com/reviewdog/reviewdog/tree/master/proto/rdf
It comes in two versions:
rdjson is a JSON that contains one object for the whole linting process, with all violations and fixes inside.
rdjsonl uses JSON Lines to print separate JSON objects for each violation.
We use the second one (rdjsonl).
The content is the same, but this format is easier to produce considering the current interface of ViolationHandler in Verible.

This is done:
There is a class named RDJsonPrinter which implements ViolationHandler interface with rule violations and fixes if they exist.
The output is interpreted by Reviewdog as expected, which can be seen here:
antmicro/gha-playground#102
Some code related to ViolationHandler has been moved from verilog to common.
It seemed like placing RDJsonPrinter inside verilog wouldn't be a good idea, so I've used the occasion to refactor these things.

This isn't done yet:
The --output_format isn't integrated with the --autofix option correctly.
Currently RDJsonPrinter will always include autofixes in the output, and will be selected only if you don't select any --autofix option.
Selecting --autofix=<patch or some other way to fix> results in producing the standard plain text output,
so If you'd like to check out this feature, please just use --output_format=rdjson.
Also, rdjson is to be renamed to rdjsonl so that it's the same name as in Reviewdog.
The name of the standard format is currently plain, and that's the default value.
I think plain, text, plaintext are worth considering.
However, I don't think this is the ultimate name and any feedback on this topic would be much appreciated.

[hzeller]

I had a quick peek.
I think I'd prefer if you have the refactoring and the additional things in separate pull requests so that it is easier to review and it also makes sense to in building up things.

A good working model is to start with the large CL that does all the things you want (like this one), then you know which parts can be separated out for quick, independent reviews.

[hzeller]

Don't forget copyright header and probalby a link to the spec we try to implement here
( https://github.com/reviewdog/reviewdog/tree/master/proto/rdf )

[hzeller]

What is the state here ? I think a good first and easy-to-review step is to do the refactoring moving things to common as there is essentially not much code-change, just moving files arond and fixing up dependencies.

Then the reviewdog output will be a nice small change on top.