feat: Filter element unions against fixed attribute values

[tefra]

📒 Description

For union fields, the parser has to attempt parsing with each type and then decide which output is the "most" successful. This is is slow and painful process, this pr adds an optimization strategy when union types have fixed attributes.

Resolves #1057

🔗 What I've Done

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💬 Comments

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🛫 Checklist

• Updated docs
• Added unit-tests
• Sample tests pass
• W3C tests pass

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 100.00%. Comparing base (cce0a16) to head (1f05835).

Additional details and impacted files

@@            Coverage Diff            @@
##              main     #1066   +/-   ##
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  Coverage   100.00%   100.00%           
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  Files          115       115           
  Lines         9238      9259   +21     
  Branches      2179      2183    +4     
=========================================
+ Hits          9238      9259   +21     

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