Move RZ_API functions out of the command handlers code

[XVilka]

DO NOT SQUASH

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

RZ_API functions have no place in the librz/core/cmd/*.c or librz/core/tui/*.c files (librz/core/cmd/cmd_api.c is an exception since it provides the API to work with commands themselves).

Thus, I moved some of the functions into better places, documenting when necessary. This is purely a code movement PR.
I didn't document ESIL functions since they are planned to be removed sometime in the future anyway.

Can be reviewed per commit.

Test plan

CI is greeen