Add very basic documentation of built-in extension structure

[mjbvz]

No description provided.

[Copilot]

Pull request overview

Adds a new top-level contributing guide under extensions/ to briefly document the expected structure of built-in extensions and the basic steps for making an extension web-enabled.

Changes:

• Introduces extensions/CONTRIBUTING.md with a “typical” built-in extension folder layout.
• Documents expected build outputs (out vs dist).
• Adds a short checklist for enabling browser/web support (browser entry, tsconfig.browser.json, esbuild.browser.mts).

Comments suppressed due to low confidence (1)

extensions/CONTRIBUTING.md:18

• The stated output structure (out for dev, dist for production) isn’t universal across built-in extensions (e.g. declarative extensions don’t have either). Consider clarifying that this applies to TypeScript-based extensions built via the gulp/esbuild pipelines, and optionally mention how main/browser typically map to out/dist.

Extensions have the following output structure:

- `out`: Output directory for development builds
- `dist`: Output directory for production builds.