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 ⚠️

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Additional details and impacted files

@@            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Outdated by #108