Convert pj to RzShell

[Rot127]

Your checklist for this pull request

• I've read the guidelines for contributing to this repository
• I made sure to follow the project's coding style
• I've documented or updated the documentation of every function and struct this PR changes. If not so I've explained why.
• I've added tests that prove my fix is effective or that my feature works (if possible)
• I've updated the rizin book with the relevant information (if needed)

Detailed description

Moves the pj command to RzShell. It also does some clean ups:

• Removes pj., because it was an alias for pj @ 0
• Removes pj.., because the JSON path function always returned NULL (never a valid value), and did not document what it does in the first place.
• Print message for user if no valid data was found at address.
• Change description of command to be more clear it only prints formatted JSON strings.
• Removes unused size/len argument.
• Adds a new test for pj.
• Adds some general documentation how to implement or find a command.

Test plan

Added

Closing issues

None, but checks a box in #1590

[XVilka]

Code is fine, with mostly documentation nitpicks