ENH: clearly highlight user-defined commands as such

[neutrinoceros]

idfx --help

shouldn't mingle builtin commands and user defined ones

[neutrinoceros]

it seems that argparse doesn't allow defining multiple subparsers, which means that currently the only way I see to implement the desired behaviour is to use https://github.com/ewels/rich-click
... but there's no way I'm switching to click just for that.

Maybe I could subclass argparse.HelpFormatter 🤔

[neutrinoceros]

argument groups (https://docs.python.org/3/library/argparse.html#argument-groups) also seem close to what I want but it doesn't seem to combine with sub parsers, that I already use