Add uiblocks addon and an xrblocks integrated sample.

[qxziuan]

This PR introduces uiblocks, a powerful new spatial UI framework addon for xrblocks. It is built on top of the @pmndrs/uikit engine and enables developers to easily create responsive, interactive, and beautiful 2D interfaces situated directly in 3D/XR space.

Key Features & Capabilities:

• Flexbox Layout Engine: Automatic UI item positioning, sizing, wrapping, and alignment (padding, gap, flow), completely eliminating the need for manual 3D coordinate math.
• Rich Aesthetics: Out-of-the-box support for glassy background panels, adjustable corner radii, borders, and smooth linear/radial gradient fills.
• Core UI Components: (1) UICard: The physical world bridge that anchors 2D UI panels into the 3D scene. It support multiple spatial behaviors such as HeadLeashBehavior, ObjectAnchorBehavior, etc. (2) UIPanel: Flexbox containers for structuring layouts. (3) UIText, UIImage, UIIcon: The UI elements.
• Built-in Interactions: Ready-to-use raycast callbacks for interactive buttons with animated state changes.

What's included in this PR:

• The complete uiblocks source library seamlessly integrated into the xrblocks build pipeline.
• Comprehensive documentation.
• A suite of isolated basic samples demonstrating individual features like cards, layouts, contents, and interactions.
• A fully assembled integration sample at samples/uiblocks/index.html showcasing a complete, interactive profile card.

[dli7319]

The package-lock is pointing to google-internal URLs.

[dli7319]

Why do we need this?

[ruofeidu]

Thank you David!

I think Xun is working on lint errors and let's review after all check have passed.

[qxziuan]

Hi @dli7319 @ruofeidu I resolved all the errors. @dli7319 I also addressed your previous feedback.

[ruofeidu]

Overall look great to me. AMAZING Major Feature Update!

Things to improve next time:

1. consistency of third-person / first person comments, capitalization.

2. xb.raycaster, xb.controllers alias to further streamline access.
   you can already use xb.scene and xb.renderer from aliases.

3. Example to compose from JSON?