Add config option for uses_only_args_and_local_vars/allow_external_class_usage

[dogweather]

Is your feature request related to a problem? Please describe.

I get these errors on my "pure" functions:

_combine is not pure because of:
	it uses external name (entries)
	it uses external name (Language)
	it uses external name (phrase)
	it uses external name (defn)
	it uses external name (lang)
	it uses external name (Definition)

IMO, these are false positives. It looks like setting allow_external_class_usage to True would resolve some of these. (?) The option was coded, but not accessible from the CLI.

Some, though, seem to be an inability to understand Python 3.10 match and type hints:

def _combine(*entries: Iterable[Entry]) -> Iterable[Entry]:
    """Combine single-entry terms with the same phrase."""

    all_entries = cast(Iterable[Entry], concat(entries))

    term_dict: dict[tuple[str, Language], list[Definition]] = defaultdict(list)

    for entry in sorted(all_entries):
        match(entry.definitions):
            case [defn]:
                key = (entry.phrase, entry.language)
                term_dict[key].append(defn)
            case _:
                raise ValueError(f"Entry expected to have exactly one definition: {entry}")

    for (phrase, lang), defns in term_dict.items():
        yield Entry(phrase, defns, lang)