refactor: modularize Schema, ObserverMap, and AttributeMap in fast-html

[janechu]

Pull Request

📖 Description

Modularize Schema, ObserverMap, and AttributeMap in @microsoft/fast-html to reduce tight coupling with TemplateElement. This is a breaking change.

Key changes:

• Moved ObserverMapConfig, ObserverMapPathEntry, ObserverMapPathNode, and ObserverMapOption from template.ts to observer-map.ts
• Moved AttributeMapConfig and AttributeMapOption from template.ts to attribute-map.ts
• Replaced Schema.jsonSchemaMap (static property) with an instance-level schemaMap and a module-level schemaRegistry export for cross-element $ref resolution
• Added Schema, AttributeMap, schemaRegistry, JSONSchema, and CachedPathMap to the public API exports
• Reversed the dependency direction: template.ts now imports from observer-map.ts and attribute-map.ts, instead of the other way around

👩‍💻 Reviewer Notes

The module dependency direction is now:

template.ts ──imports──▶ observer-map.ts (config types)
template.ts ──imports──▶ attribute-map.ts (config types)
template.ts ──imports──▶ schema.ts
observer-map.ts ──imports──▶ schema.ts (types only)
attribute-map.ts ──imports──▶ schema.ts (types only)
utilities.ts ──imports──▶ schema.ts (schemaRegistry for cross-element $ref)

📑 Test Plan

All 873 existing tests pass across Chromium, Firefox, and WebKit. No new tests were needed as the refactor preserves all existing behavior — only the internal module boundaries and export surface changed.

✅ Checklist

General

• I have included a change request file using $ npm run change
• I have added tests for my changes.
• I have tested my changes.
• I have updated the project documentation to reflect my changes.
• I have read the CONTRIBUTING documentation and followed the standards for this project.