Support xunit v3

[martincostello]

Initial attempt at getting support for xunit v3.

Will likely require much more involved changes to be able to support v2 and v3 simultaneously (e.g. a new package). Visual Studio integration does not appear to work, and that's making it difficult to debug the fact that the message sink doesn't appear to be working.

Resolves #717.

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 98.52%. Comparing base (6cae7de) to head (142e8a8). Report is 83 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #718      +/-   ##
==========================================
+ Coverage   97.78%   98.52%   +0.73%     
==========================================
  Files          15       15              
  Lines         271      271              
  Branches       37       37              
==========================================
+ Hits          265      267       +2     
+ Misses          3        2       -1     
+ Partials        3        2       -1     

Flag	Coverage Δ
linux	97.04% <ø> (ø)
macos	97.78% <ø> (ø)
windows	98.52% <ø> (+1.47%)	:arrow_up:

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[martincostello]

It's shipped as stable: xunit v3 1.0.0

[martincostello]

Superseded by #804.