Add help command that displays list of commands

[yotamN]

Together with some refactor to make it possible, I've added an help message (and an unload message that was declared but not implemented).

I've developed it over my other patch of auto java perform because it touched the same code area and I wanted to minimize merge conflicts later, hope it's okay.

[yotamN]

Fixed everything and rebased

[oleavr]

Thanks! Conflicts ⚔️

[yotamN]

Fixed

[oleavr]

Conflicts ⚔️

[yotamN]

rebased again

[starplanet]

There is a bug that _repl_magic.py is not listed in meson.build file. It will cause frida to fail to compile