Add Flutter Web Support

[leehack]

#29

[codecov]

Codecov Report

Attention: Patch coverage is 93.75000% with 1 line in your changes missing coverage. Please review.

Project coverage is 45.34%. Comparing base (b448a65) to head (ab601e3).
Report is 2 commits behind head on main.

Files with missing lines	Patch %	Lines
lib/src/client/streamable_https.dart	93.75%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #32      +/-   ##
==========================================
- Coverage   45.47%   45.34%   -0.14%     
==========================================
  Files          15       15              
  Lines        2876     2869       -7     
==========================================
- Hits         1308     1301       -7     
  Misses       1568     1568              

Flag	Coverage Δ
unittests	45.34% <93.75%> (-0.14%)	⬇️

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[leehack]

The example app runs fine on other platforms, but there are still some issues on the web.