Support numeric constraints for Decimal types

[cocorigon]

This is my first time playing around with C, so please feel free to tell me if more micro-optimizations can be achieved. Same for compiler hints, notably for the MS_INLINE and MS_NOINLINE near ms_check_decimal_constraints(...): I implemented it in a manner similar to what was done for floats for example, though I must admit that I did not quite understand the pattern for when to use each.

Also, I added a DevContainer for easier setup for newcomers to the project.

Closes #683.

[Siyet]

Hi @cocorigon! I've run into the same issue (#683) in production - Annotated[Decimal, Meta(gt=0)] raises TypeError when used with schema_components.

Are you still planning to rebase this PR and continue working on it? If not, I'd be happy to pick it up - would open a fresh PR based on your approach, with credit of course.

No rush, just want to make sure the effort isn't duplicated.

[cocorigon]

Hey @Siyet! I don't really do much Python anymore, so no, I will not work on this PR again in the near future.

So feel free to use this code as you wish!

[Siyet]

Hi @cocorigon, thanks again for the original work here and for the explicit handover note above.

I've picked this up in #1006, which builds on the same approach (C-level constraint check, multiple_of via Decimal arithmetic) and is rebased against the current src/msgspec/ + tests/unit/ layout. Closing this one as superseded so review attention can stay on a single thread. Credit to you is in the #1006 description.

Closes #683 will move to #1006.