Fix CommandArgumentEnumerator behavior for .NET 8

[thomaslevesque]

Fixes #541

There's been a breaking change in .NET 8. The behavior for IEnumerator.Current, which has always been undefined when called after MoveNext() returned false, has changed from "return the last value" to "throw an exception". So instead of calling _enumerator.Current every time Current is invoked, we now store the value of _enumerator.Current after _enumerator.MoveNext() returned true.

I also added the net8.0 TFM to the unit tests to validate the behavior is now correct on .NET 8.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (0726df9) 82.34% compared to head (5b1d19f) 77.86%.
Report is 2 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #544      +/-   ##
==========================================
- Coverage   82.34%   77.86%   -4.49%     
==========================================
  Files         105      105              
  Lines        3308     3311       +3     
  Branches        0      717     +717     
==========================================
- Hits         2724     2578     -146     
+ Misses        584      580       -4     
- Partials        0      153     +153     

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[thomaslevesque]

Closing in favor of #542