perf(core): make classDeclaration validation more CPU efficient

[mttrbrts]

This change improves the algorithmic complexity of validating ClassDeclarations from O(n^2) to O(n). This gives a significant improvement for models with large numbers of class declarations.

Previously, we would compare each ClassDeclaration name with every other to detect duplicates. The implementation uses ES6's Set structure to more efficiently count duplicates.

Flags

• We still use a more expensive lookup to find the name of duplicate (for use in the error message), but only in the case where there is a duplicate definition.

Author Checklist

• Ensure you provide a DCO sign-off for your commits using the --signoff option of git commit.
• Vital features and changes captured in unit and/or integration tests
• Commits messages follow AP format
• Extend the documentation, if necessary
• Merging to main from fork:branchname