SepReader: Add async Support

[nietras]

No description provided.

[codecov]

Codecov Report

Attention: Patch coverage is 83.99247% with 85 lines in your changes missing coverage. Please review.

Project coverage is 97.50%. Comparing base (77c0499) to head (991157e).
Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
src/Sep/SepReader.IO.Async.cs	78.67%	44 Missing and 1 partial ⚠️
src/Sep/SepReaderExtensions.IO.Async.cs	28.00%	35 Missing and 1 partial ⚠️
src/Sep/SepReader.IO.Sync.cs	98.57%	2 Missing and 1 partial ⚠️
src/Sep/SepReader.cs	88.88%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #219      +/-   ##
==========================================
- Coverage   99.58%   97.50%   -2.09%     
==========================================
  Files          47       51       +4     
  Lines        3613     3886     +273     
  Branches      400      433      +33     
==========================================
+ Hits         3598     3789     +191     
- Misses         13       93      +80     
- Partials        2        4       +2     

Flag	Coverage Δ
Debug	97.06% <81.92%> (-2.36%)	⬇️
Release	97.51% <81.02%> (-2.34%)	⬇️
macos-latest	90.32% <81.92%> (-1.85%)	⬇️
ubuntu-latest	97.19% <81.92%> (-2.37%)	⬇️
windows-latest	97.19% <81.92%> (-2.37%)	⬇️

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