[heft-config-file] Add precompiled JSON schema validator support

[Copilot]

Schema publishers currently compile AJV validators at runtime. This PR introduces build-time precompilation and a path for configuration loaders to use the generated validators while continuing to publish JSON schemas.

• Generation: Add a built-in Heft task plugin that emits standalone CommonJS validators from *.schema.json files. Generated modules resolve AJV runtime dependencies through @rushstack/node-core-library.
• Runtime: Add jsonSchemaValidator to heft-config-file and have Heft plugin option-schema loading prefer a generated validator when present.

new ProjectConfigurationFile({
  projectRelativeFilePath: 'config/my-plugin.json',
  jsonSchemaValidator: require('./schemas/my-plugin.validator.cjs')
});

This establishes the precompilation path; schema-publishing projects are not yet migrated to run the new task.