FR: All commands to have optional structured output, eg. --json

[neongreen]

Is your feature request related to a problem? Please describe.
I want to be able to say things like

CHANGE_ID=$(jj new main --json | jq .change_id)

or

AUTHOR=$(jj show $commit --json | jq .author.name)

This comes up often when doing any scripting that involves repo operations.

Describe the solution you'd like
Add --json to the most popular commands. It would cause jj to stdout a json object with the "result" of whatever the command does.

It's ok if

• it's just one or two commands at first,
• and it's also ok if the JSON object is merely a subset of the result,
• and it's also ok to fail to output any json if the command fails for whatever reason.
  • (but ideally the output would be {"error": ..., "details": {...}} or smth of course)

Describe alternatives you've considered
I haven't really. I mean, the backend/api idea is an alternative in a way, since then I just do curl jj/show instead of jj show --json, but I'd say the --json bit is actually a stepping stone towards the api.

[yuja]

Some related issues:

• Bikeshedding how we use/offer templates #3262
• FR: Add jj api command to establish structured interprocess communication for seamless IDE integration #3219
• FR: make jj commands more composable #3814

[joyously]

Also related: FR: Add a json function in template language for serializing strings