feature(bot): add CLI argument parsing helper

[davla]

Why

@Cruifer is starting to expect and rely on help texts in custom bot scripts. That poses a number of problems at present:

• We don't always have a -help flag to show the script __doc__, even though we do have a docstring most of the times.

• The actual parameter names and what's contained in the script can mismatch, as the whole argument parsing is implemented manually

• Since we have to use with Pywikibot's terrible CLI option synthax (-name:<value>), in the few places where we have used the Python standard library we have inconsistency, as we have a sane syntax (e.g. --name <value>).

What

After looking around, I couldn't find any decent library that would allow CLI named parameters to be split on : and start with a single - (henceforth referred to as Pywikibot's terrible CLI option syntax). I hence had to implement something myself.

PcwCliArgs is yet another ArgumentParser-looking CLI argument parsing utility. You add options, flags and positional arguments, and then you parse them into a dictionary. If the -help flag is found, it prints an auto-generated help text and exits.

I wanted to do something with dataclasses and reflection, but apparently Python's class attributes can't have their own docstrings, so here we are with a builder interface.

The only issue is that we'd need the current directory in the PYTHONPATH environment variable. That's not hard to achieve though.

[flavio-a]

I think I had some kind of objection last time I reviewed thiw but I can't remember it right now, so I'll approve before it comes back to mind.