Add sources_operators

[vxgmichel]

An attempt to address issue #95 by adding another *operator decorator dedicated to operators that can take zero, one or many async iterables as arguments.

There are two issues with this attempt:

• the new decorator does not support operator with keyword arguments, so an empty ziplatest() is still not supported.
• this break proper type inference for the merge and zip operators, as seen here: https://github.com/vxgmichel/aiostream/pull/96/files#diff-013a68594b5bfcc0727109dc487131baf28839aa529c426944bdfb467edbc02cR265

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 94.02985% with 4 lines in your changes are missing coverage. Please review.

Project coverage is 98.95%. Comparing base (daf6524) to head (a0d0790).

Files	Patch %	Lines
aiostream/core.py	93.10%	4 Missing :warning:

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Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #96 +/- ## ========================================== + Coverage 98.90% 98.95% +0.05% ========================================== Files 15 15 Lines 1004 1053 +49 ========================================== + Hits 993 1042 +49 Misses 11 11 ```

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[vxgmichel]

Outdated by #108