Update tiktoken regexes

[stephentoub]

This updates two of the regexes to match the changes made in openai/tiktoken@9f7f69d.

On .NET Core, these changes are mostly nops, as the main thing they're doing is changing some loops to be atomic, and the auto-atomicity logic in the regex optimizer was already noticing that could be done and doing it automatically. On .NET Framework, it's a bigger deal, as those loops will now be atomic where they weren't previously.

If nothing else, it keeps the regexes in sync with the reference implementation.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 68.78%. Comparing base (be1e428) to head (14fff61).
Report is 5 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #7255   +/-   ##
=======================================
  Coverage   68.77%   68.78%           
=======================================
  Files        1462     1463    +1     
  Lines      272261   272288   +27     
  Branches    28176    28177    +1     
=======================================
+ Hits       187254   187297   +43     
+ Misses      77764    77748   -16     
  Partials     7243     7243           

Flag	Coverage Δ
Debug	68.78% <ø> (+<0.01%)	⬆️
production	63.28% <ø> (+<0.01%)	⬆️
test	89.04% <ø> (+<0.01%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Files with missing lines	Coverage Δ
...Microsoft.ML.Tokenizers/Model/TiktokenTokenizer.cs	77.91% <ø> (+0.08%)	⬆️

... and 10 files with indirect coverage changes

[stephentoub]

/ba-g unrelated torchsharp crash