feat: Bump observablehq runtime support

[GordonSmith]

Checklist:

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  • The commit message includes a "fixes" reference if appropriate.
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Testing:

[GordonSmith]

remove comments code

[Copilot]

Pull Request Overview

This PR upgrades the observablehq runtime support by transitioning from an older implementation to a newer, streamlined approach. The changes remove dependencies on top-level await plugins and related build configurations while adding a custom runtime implementation.

Key changes include:

• Simplified Vite configuration by removing top-level await plugin and related build settings
• Added a new runtime entry point for notebook kit functionality
• Updated test files to use the new runtime implementation and simplified test structure

Reviewed Changes

Copilot reviewed 20 out of 22 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
packages/observablehq-compiler/vite.config.ts	Removes top-level await plugin configuration and adds new runtime build entry
packages/observablehq-compiler/tsconfig.json	Includes new runtime source file in TypeScript compilation
packages/observablehq-compiler/tests/system-guide.txt	Adds new test content for system guide functionality
packages/observablehq-compiler/tests/index-notebookkit.ts	Refactors to use new runtime implementation and simplified test structure
packages/observablehq-compiler/tests/index-notebookkit.js	New JavaScript test file with embedded HTML content
packages/observablehq-compiler/tests/albers-usa-projection.txt	Adds new test content for projection functionality

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