AVRO-4350: [JavaScript] Remove underscore dependency

[svozza]

What is the purpose of the change

AVRO-4350

Follow-up to #3970, which restored Underscore to fix broken clean installations of avro-js.

This removes the Underscore usage from the deprecated validator by replacing collection operations with native JavaScript APIs and using the existing util.isDeepStrictEqual API for structural comparisons.

The legacy Nodeunit validator tests have been moved into the main Mocha suite. Obsolete Nodeunit and Grunt test infrastructure has also been removed.

Verifying this change

Added coverage for the deprecated validator, including:

• Primitive, record, enum, array, map, union, and protocol validation
• Boxed primitive and special number behavior
• Equivalent and conflicting repeated named records
• Deeply nested repeated record definitions
• Equivalent and conflicting repeated named enums
• Null-safe protocol validation

Verified with:

• ./build.sh test: 414 tests passed
• npm run lint
• A clean npm ci with Underscore absent
• Packaging the modified source, installing the resulting tarball in a clean project, and successfully requiring avro-js

Documentation

• Does this pull request introduce a new feature? no
• If yes, how is the feature documented? not applicable