Added opt-in features

[glen-84]

Summary of the changes (Less than 80 chars)

• Added opt-in features.

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❓ Questions:

• Please check the schema error definitions in ErrorHelper.cs.
  • Should I be setting an error code or specified by?
• I'm returning one error for each application of the directive, is that correct?
  • In theory, we could be including information about the directive syntax position, arguments, etc.

📓 Spec notes:

• Should we explicitly disallow empty and whitespace-only feature strings? *
• Should we disallow whitespace or other characters within feature names, or leave it completely open? *
• I've used includeOptIn instead of includeRequiresOptIn for introspection, as it seemed more clear, WDYT?

* stability strings as well.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 73.91%. Comparing base (28e169d) to head (8e5e2d1).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #7652      +/-   ##
==========================================
+ Coverage   73.82%   73.91%   +0.08%     
==========================================
  Files        2918     2929      +11     
  Lines      151114   151541     +427     
  Branches    17555    17602      +47     
==========================================
+ Hits       111565   112006     +441     
+ Misses      33726    33706      -20     
- Partials     5823     5829       +6     

Flag	Coverage Δ
unittests	73.91% <ø> (+0.08%)	⬆️

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