Expose instruction encoding, CSR bitfield information as JSON

[Timmmm]

Currently there isn't an easy way to extract useful information from the Sail model like:

• Instruction encoding patterns
• Instruction assembly format
• CSR field information

This information is really useful in some contexts (e.g. coverage), and there's definitely a desire for it - hence the UDB effort.

But I would much rather use the Sail model than UDB, so it would be nice to get this information in easily digestible form (i.e. JSON).

There is the JSON doc bundle, but it's targeted at embedding Sail into Asciidoc.

I think this is basically what @ThinkOpenly has been working on, but I don't think we have an issue for it. @ThinkOpenly can you say how far you got, and what it would take to get a JSON file with all this information building in sail-riscv's CI?