feat: Add help_formatter system wherein help text rendering can be customized.

[DanCardin]

This enables one to include or exclude things like default values, or choices. Notably changes to include default values in help text by default.

Fixes #125

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A kitchen-sink style example of most of the available options, used together.

from cappa import parse, HelpFormatter, Arg

class Command:
    foo: Annotated[str, Arg(help="Help text.", deprecated=True)] = "foo"

def deprecated(arg: Arg) -> str | None:
    if arg.deprecated:
        return "Deprecrated"
    return None

parse(Command, help_formatter=HelpFormatter(arg_format=("{default}", "{help}", deprecated), default_format="(Default `{default}`)")

Resulting in something like (Default foo) Help text. Deprecrated.

[euri10]

I read that quite quicklu, I had just one comment, I'll try test it later today to get more of a feel for it, but so far I love it, simple