[typing] Inconsistent typing in the work decorator

[rodrigogiraoserrao]

See this discussion about the usage of Decorator versus Decorator[..., ReturnType] in src/textual/_work_decorator.py.

EDIT: Last time I tackled this (and opened PR #3862) I ended up hitting what looked like a mypy issue but might as well just be something I didn't get right.
I did try to outsource the fix to Twitter and a Discord server but no one was able to help me.
Independently from me, PR #3937 does similar work that would also fix this issue.

[willmcgugan]

@rodrigogiraoserrao I'm assuming this is a quick fix?

[rodrigogiraoserrao]

Depends.
When it comes to typing, what do we use as the ground truth? mypy, pylance, or another typechecker?

[willmcgugan]

Ideally we can get them to agree. If not, Mypy.

[rodrigogiraoserrao]

I was exploring a fix to the typing issues that then triggered new typing issues in other places of the codebase where the decorator work is used.

Work on this surfaced a possible mypy bug.
I've created a standalone gist which seems to contain this possible bug: https://mypy-play.net/?mypy=latest&python=3.12&gist=c451dedbb5ec867b98fcfec6aa24d698
I also asked for help on Twitter: https://twitter.com/mathsppblog/status/1734247854854599035