SSE using @microsoft/fetch-event-source so we can post and control more

[samuelcolvin]

means:

• SSE connections don't automatically reconnect
• we can use methods other than GET
• we can include headers for auth

Partly resolves #171

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (d3a8e6c) 95.65% compared to head (6578cb8) 95.67%.
Report is 3 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #193      +/-   ##
==========================================
+ Coverage   95.65%   95.67%   +0.02%     
==========================================
  Files          14       14              
  Lines         943      948       +5     
==========================================
+ Hits          902      907       +5     
  Misses         41       41              

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