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 95.00000% with 3 lines in your changes missing coverage. Please review.
✅ Project coverage is 94.83%. Comparing base (53f9d49) to head (9b98d32).
⚠️ Report is 152 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%   94.83%   -0.90%     
- Complexity     1773     1891     +118     
============================================
  Files           154      176      +22     
  Lines          4586     4974     +388     
============================================
+ Hits           4390     4717     +327     
- Misses          196      257      +61     

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[oojacoboo]

Thanks for putting this together @oprypkhantc!

I'm thinking we should deprecate support for InputType::isUpdate as part of this PR and update the docs to direct the Undefined "type" as the de facto way of handling update functionality, where nullable support is needed.

[oojacoboo]

@oprypkhantc can we get this merged?

[oprypkhantc]

Hey @oojacoboo. Sorry, I forgot to update you - I no longer work directly for the company I used to and no longer work with PHP; I might occasionally come back and fix things here and there though :)

I updated the PR with deprecation of update: flag and updated the docs, but feel free to take over if I missed anything; also if you need me for anything you can tag my personal account at @autaut03, I'll see a notification for that one ❤️

[oojacoboo]

@oprypkhantc @autaut03 Thanks for the update, both on the PR and your personal situation. It's been a pleasure working with you on graphqlite. All the best in your new endeavors!