fix: support content and OpenAPI keywords

[MohitSAGAR11]

Closes #294

Summary

A schema that uses a keyword the package doesn't know about makes jsonSchemaErrors throw Encountered unsupported keyword .... This adds support for the keywords the standard dialects and the OpenAPI dialects can put in the AST.

Content keywords (draft-07 and up)

contentMediaType, contentEncoding, and contentSchema are annotation-only, but they weren't registered. They're now registered as annotations, alongside comment, default, description, and examples.

OpenAPI keywords

The OpenAPI dialects define their own keyword URIs:

• The annotation-only keywords discriminator, example, externalDocs, xml, and nullable (OpenAPI 3.0) don't affect validation and are registered as annotations.
• The OpenAPI 3.0 type keyword asserts types like the standard type keyword, so the type/const/enum error handler now recognizes it and produces the same messages (including the null type added by nullable).

OpenAPI 3.1 and 3.2 reuse the same keyword URIs, so they're covered too.

Tests

• src/test-suite/tests/content.json — the content keywords are annotations across draft-07, 2019-09, and 2020-12.
• src/openapi.test.js — OpenAPI 3.0/3.1/3.2 type assertions, nullable, annotation keywords, and annotation-only schemas.

npm test (1338 passing), npm run lint, and npm run type-check all pass.

[MohitSAGAR11]

Hi @jdesrosiers, thanks again for the quick reply on #294.

While this PR is up, I wanted to check in: is there anything you'd like help with, or a direction you'd prefer I work toward? I'm happy to keep doing what I've been doing — finding and reporting issues, then opening a PR for the ones you approve — but I didn't want to assume. Let me know what would be most useful.