Add a IsNotA expression as a symmetry to the existing IsA expression.

[sukei]

I created a IsInstanceOf expression for my own project that is roughly the same implementation as the new IsA expression. But I also use a IsNotInstanceOf which have no equivalent yet. This PR add the IsNotA expression that is the exact inverse of the existing IsA implementation. I also rewrite the violations message to better align with the format of other expressions and that works better with the because suffix that may be append.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 97.90%. Comparing base (75ccf07) to head (c18d341).
⚠️ Report is 4 commits behind head on main.

Additional details and impacted files

@@             Coverage Diff              @@
##               main     #573      +/-   ##
============================================
+ Coverage     97.89%   97.90%   +0.01%     
- Complexity      660      664       +4     
============================================
  Files            83       84       +1     
  Lines          1900     1911      +11     
============================================
+ Hits           1860     1871      +11     
  Misses           40       40              

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[fain182]

Great, thanks!