[api-minor] Convert documentInfo to return any "Custom" data in a Map

[Snuffleupagus]

The "Custom" entry is a feature that, outside of a single unit-test, isn't used anywhere in the PDF.js code-base.
Besides changing this data to use a Map, rather than a regular Object, the validation can be moved since it's not necessary when a white-listed type has been encountered.

[codecov-commenter]

Codecov Report

❌ Patch coverage is 25.00000% with 3 lines in your changes missing coverage. Please review.
✅ Project coverage is 90.20%. Comparing base (49ff3dc) to head (3707b66).

Files with missing lines	Patch %	Lines
src/core/document.js	25.00%	3 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master   #21828      +/-   ##
==========================================
+ Coverage   90.18%   90.20%   +0.01%     
==========================================
  Files         264      264              
  Lines       67312    67310       -2     
==========================================
+ Hits        60707    60715       +8     
+ Misses       6605     6595      -10     

Flag	Coverage Δ
browsertest	66.30% <0.00%> (-0.03%)	⬇️
fonttest	8.92% <ø> (ø)
integrationtest	69.42% <25.00%> (+0.01%)	⬆️
unittest	58.37% <25.00%> (-0.01%)	⬇️
unittestcli	56.94% <25.00%> (-0.02%)	⬇️

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[timvandermeij]

Thank you!