Add pubsub support for SSE and WebSocket

[miguelgrinberg]

This change adds a small pubsub framework that applications can use to push updates under a set of topics to which clients can subscribe.

[codecov-commenter]

Codecov Report

❌ Patch coverage is 0% with 18 lines in your changes missing coverage. Please review.
✅ Project coverage is 98.86%. Comparing base (680cbef) to head (ca26486).
⚠️ Report is 23 commits behind head on main.

Files with missing lines	Patch %	Lines
src/microdot/pubsub.py	0.00%	18 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##              main     #320      +/-   ##
===========================================
- Coverage   100.00%   98.86%   -1.14%     
===========================================
  Files           15       16       +1     
  Lines         1564     1582      +18     
  Branches       280      284       +4     
===========================================
  Hits          1564     1564              
- Misses           0       18      +18     

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