chore: remove deprecated declarative event e support

[janechu]

Pull Request

📖 Description

Remove deprecated support for bare e in FAST declarative event handler arguments.

This makes $e and $c the only special event-binding arguments, removes the deprecated warning/runtime compatibility path, deletes the deprecated warning fixture, and updates the migration and declarative docs for the breaking change.

🎫 Issues

• Closes FAST HTML remove support for e as an argument in event handlers #7371

👩‍💻 Reviewer Notes

The main changes are in packages/fast-element/src/declarative/:

• bare e is no longer treated as a special event argument
• TemplateParser.hasDeprecatedEventSyntax and the related warning path were removed
• declarative fixtures and docs now use $e

I also excluded declarative fixture templates.html/generated index.html files from the repo's Biome changed-file checks because those artifacts are whitespace-sensitive and Biome's explicit file-path mode was flagging generated fixture HTML that is intentionally not formatter-safe.

📑 Test Plan

• npm run build
• npm run test
• npm run biome:check
• npm run checkchange

✅ 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.

⏭ Next Steps

• None.