Fix bash quoting, add type hints, code cleanup

[pmhahn]

I found your project while looking for how to get bash completion automatically from ArgumentParser. Sadly Ubuntu 24.04 ships a broken version, so I had a look at your GitHub repository. Great work. Sadly there are several pending open issues and other PRs, so I hope you will find some time to work on this again soon.

1. While looking at the code I noticed that type-annotations were incomplete. As I'm a huge fan of type-annotated code myself, I took my time to add it to all functions.[^1]
2. I upgraded the old annotations to use list[…] instead of typing.Lits[…] and such.
3. As usual mypy found some minor issues, which I also fixed. See individual commits at the front.
4. While looking at the generated code shellcheck found many issues, mostly related to missing quotes. I fixed the code, which generates those. See individual commits in the middle.
5. __init__.py is quiet large and contains mixed Python/shell code. Maybe it might be a good idea to move the shell-code to separate files; see last commit.

[^1]: For complete type annotations typing.Protocol from Python 3.8 (EoL last year) is needed. Therefor I bumped the minimum supported version to 3.8 not that 3.14 got released. By using from __future__ import annotations and if TYPE_CHECKING: the code still runs with 3.8. Using from collections.abc import … instead of from typing import … works only since Python 3.9 (EoL this month). Raising the minimum version to 3.9 thus might be preferred.

If you prefer I can send you smaller pull requests only doing one thing at a time. But before I spend more time on this, I'd like to get your feedback if such changes are preferred or not.

[pmhahn]

Thanks @pmhahn - apologies for not reviewing sooner.

Would very much prefer solely the quote (and any bugfixes) in a separate PR - which I'd be happy to merge; and the rest (stylistic stuff) can be debated in other PR(s).

Sorry for the delay. I have force-pushed my branch only containing the 2 commits fixing quoting.

How do you want my other changes? I can try to split it into smaller chunks by topic if you prefer.