Add VS Code grammar package for Astral templates

[dannote]

Summary

• Add an optional packages/vscode extension package for .astral files.
• Implement a minimal TextMate wrapper grammar that highlights setup blocks as Elixir and delegates template bodies to Phoenix HEEx.
• Add headless TextMate snapshot tests for setup blocks, no-setup templates, dynamic route setup, and style/script blocks.
• Recommend the Astral, Phoenix, and ElixirLS extensions from generated starter projects and the example site.
• Document the editor tooling direction and long-term Elixir/GenLSP plan.

Notes

The VS Code package is intentionally isolated from the core Astral workflow. Core users and mix ci do not need Node.js for this package; npm tooling is only used when developing or publishing the optional VS Code extension.

Validation

• npm --prefix packages/vscode run test:grammar
• npm --prefix packages/vscode run package
• mix ci