Introduce Undefined::VALUE

[oprypkhantc]

Closes #726

Tests are incomplete, but otherwise this is ready for review

From my perspective, the next step would be deprecating isUpdate completely - what do you think of that? I didn't include it in this PR as there may be value in trying out the Undefined approach first for a larger chunk of users. For us though, this approach has been good.

[codecov-commenter]

Codecov Report

❌ Patch coverage is 94.54545% with 3 lines in your changes missing coverage. Please review.
✅ Project coverage is 94.79%. Comparing base (53f9d49) to head (28ab03d).
⚠️ Report is 125 commits behind head on master.

Files with missing lines	Patch %	Lines
src/Mappers/Parameters/TypeHandler.php	88.88%	3 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##             master     #764      +/-   ##
============================================
- Coverage     95.72%   94.79%   -0.94%     
- Complexity     1773     1866      +93     
============================================
  Files           154      176      +22     
  Lines          4586     4916     +330     
============================================
+ Hits           4390     4660     +270     
- Misses          196      256      +60     

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