SepWriter: Add async support (C# 13+ only)

[nietras]

No description provided.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 99.65%. Comparing base (f92b65b) to head (1118960).
Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #224      +/-   ##
==========================================
+ Coverage   99.63%   99.65%   +0.01%     
==========================================
  Files          51       54       +3     
  Lines        3876     4019     +143     
  Branches      433      457      +24     
==========================================
+ Hits         3862     4005     +143     
  Misses         11       11              
  Partials        3        3              

Flag	Coverage Δ
Debug	99.45% <99.63%> (-0.01%)	⬇️
Release	99.82% <100.00%> (+<0.01%)	⬆️
macos-latest	92.95% <100.00%> (+0.25%)	⬆️
ubuntu-latest	99.60% <100.00%> (+0.01%)	⬆️
windows-latest	99.60% <100.00%> (+0.01%)	⬆️

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