feat: move declarative html into fast-element

[janechu]

Pull Request

📖 Description

Move the declarative HTML runtime from @microsoft/fast-html into @microsoft/fast-element under a dedicated @microsoft/fast-element/declarative.js entrypoint.

This PR:

• moves the declarative runtime, fixtures, scripts, Playwright configs, and package docs into packages/fast-element
• updates internal consumers, website/API docs generation, and the WebUI integration workflow to use the new declarative subpath
• removes the packages/fast-html workspace and its legacy changefiles

Closes #7314

👩‍💻 Reviewer Notes

Review the migration in roughly this order:

1. packages/fast-element/src/declarative.ts and packages/fast-element/package.json for the new public entrypoint and export wiring.
2. packages/fast-element/test/declarative/ and packages/fast-element/scripts/declarative/ for the moved fixture/test infrastructure.
3. sites/website/scripts/generate-docs.cjs and sites/website/src/docs/3.x/ for the new declarative API/docs publication path.

📑 Test Plan

• npm run build
• npm run checkchange
• npm run test:chromium:declarative -w @microsoft/fast-element
• cd packages/fast-element && npx playwright test --config=playwright.declarative.config.ts test/declarative/fixtures/ecosystem/deprecated-event-warning/deprecated-event-warning.spec.ts --workers=1
• cd packages/fast-element && npx playwright test --config=playwright.declarative.config.ts test/declarative/fixtures/extensions/attribute-map-naming-strategy/attribute-map-naming-strategy.spec.ts --project=webkit --workers=1
• npm run test in the shared CLI environment reached the declarative Playwright matrix and failed intermittently while another repo was reusing port 5174; the focused reruns above passed.

✅ 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.