refactor: Address PR feedback for vocabulary bitset optimization

This commit addresses all maintainer feedback on PR #2040:

API improvements:
- Use initializer_list constructor instead of chained insert() calls
- Use brace initialization {} for construction
- Change insert() signature: insert(uri, required) instead of pair
- Add enum-based overloads (contains/insert/get with Known enum)
- Rename method from find() to get() for clarity

Implementation improvements:
- Rename jsonschema_vocabularies.cc to vocabularies.cc
- Reorder vocabulary checks from newest to oldest (2020-12 first)
- Rename short variable names (it -> iterator)
- Move inline comments above their respective lines
- Remove merge() method and update all usages

Test updates:
- Replace all merge() calls with combined vocabulary constants
- Add VOCABULARIES_2019_09_APPLICATOR_AND_VALIDATION
- Add VOCABULARIES_2020_12_APPLICATOR_AND_VALIDATION
- Add VOCABULARIES_2020_12_UNEVALUATED_AND_APPLICATOR
- Add VOCABULARIES_2020_12_APPLICATOR_UNEVALUATED_AND_VALIDATION
- Update test macros to use get() instead of at()
- Update walker_test to use get() instead of find()

Documentation:
- Add TODO comment documenting future optimization opportunities
- Note priority areas: alterschema rules, bundle.cc, frame.cc, official_walker.cc
- Document potential for orders of magnitude performance improvement

Cleanup:
- Remove benchmark files per maintainer request

All changes maintain backward compatibility while providing optimized
enum-based API for known vocabularies.

Author: syedazeez337