Implemented a command catalog exporting as a json

[moste00]

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• I've read the guidelines for contributing to this repository.
• I made sure to follow the project's coding style.
• I've documented every RZ_API function and struct this PR changes.
• I've added tests that prove my changes are effective (required for changes to RZ_API).
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Detailed description

This implements suggestion C1 of #6543, namely, it exports the full-command tree as a json tree. The export can be triggered by both a command line option --cmd-catalog json, and via an internal help statement like ?*, named ?+j.

Test plan

For now, none. I plan to add it soon. The following will be implemented:

(1) A test that asserts the top-level number of children is the same as the number of commands outputed by ?

(2) Manual comparison of the output against the yaml description of the commands, by both humans and several LLMs

Closing issues

None, small part of addressing #6543.

[moste00]

@notxvilka please review ?