Add SepParserAvx256To128CmpOrMoveMaskTzcnt and change SepParserFactory order

[nietras]

Not faster but around 19 GB/s so actually faster than AVX-512 still due to mask code gen issues. Avx2 and Avx512To256 faster.

[codecov]

Codecov Report

Attention: Patch coverage is 66.66667% with 3 lines in your changes missing coverage. Please review.

Project coverage is 99.58%. Comparing base (b364d43) to head (1672554).
Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
src/Sep/Internals/SepParserFactory.cs	66.66%	0 Missing and 3 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #296      +/-   ##
==========================================
+ Coverage   99.56%   99.58%   +0.02%     
==========================================
  Files          54       54              
  Lines        4369     4370       +1     
  Branches      508      511       +3     
==========================================
+ Hits         4350     4352       +2     
+ Misses         14       12       -2     
- Partials        5        6       +1     

Flag	Coverage Δ
Debug	99.26% <44.44%> (-0.03%)	⬇️
Release	99.60% <44.44%> (-0.10%)	⬇️
macos-latest	93.31% <22.22%> (-0.03%)	⬇️
ubuntu-latest	99.42% <44.44%> (-0.03%)	⬇️
windows-latest	99.42% <44.44%> (-0.03%)	⬇️

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