feat: implement undefinedAsNull keyword for enum type

[alexander-akait]

This PR contains a:

• bugfix
• new feature
• code refactor
• test update
• typo fix
• metadata update

Motivation / Use-Case

part of webpack/webpack#17339

Breaking Changes

No

Additional Info

Only for enum

We need backport it for v3 too
Also we need to implement this for tooling

[codecov]

Codecov Report

Patch coverage: 100.00% and project coverage change: +0.04 🎉

Comparison is base (6d5a65f) 97.43% compared to head (7c5a40e) 97.48%.

❗ Current head 7c5a40e differs from pull request most recent head 10443c3. Consider uploading reports for the commit 10443c3 to get more accurate results

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #175      +/-   ##
==========================================
+ Coverage   97.43%   97.48%   +0.04%     
==========================================
  Files           6        7       +1     
  Lines         741      755      +14     
  Branches      318      321       +3     
==========================================
+ Hits          722      736      +14     
  Misses         17       17              
  Partials        2        2              

Impacted Files	Coverage Δ
src/ValidationError.js	96.52% <100.00%> (+0.02%)	⬆️
src/keywords/undefinedAsNull.js	100.00% <100.00%> (ø)
src/validate.js	100.00% <100.00%> (ø)

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