feat: support async declarative template strings

[janechu]

Pull Request

📖 Description

Adds support for async declarative template strings in declarativeTemplate({ callback }). The callback now receives a templateStringResolver that accepts a string | Promise<string> containing an <f-template> element and resolves it through the existing declarative template path.

This also adds source-level coverage, an ecosystem async-template-string fixture, updated API/docs/website generated content, size docs, and a change file.

👩‍💻 Reviewer Notes

Please focus review on the callback resolver semantics and diagnostic behavior: callbacks must return or await the resolver, and callbacks that complete without resolving now reject with a clear diagnostic instead of hanging.

Root npm run test was run. It fails only with unrelated Firefox caveats after exact reruns: the unmodified hydration Firefox failure (hydration.pw.spec.ts:179:5, element.$fastController is undefined) plus Firefox fixture flakes that pass on exact rerun/full fast-test-harness rerun.

📑 Test Plan

• npm ci
• npx playwright install --with-deps
• npm run build
• npm run biome:check
• npm run checkchange
• Focused fast-element TypeScript/lint/docs/API/fixture tests passed
• Root npm run test was run with the unrelated Firefox caveats noted above

✅ 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.