Examples: Add WebGPU Contact Shadows

[querielo]

This pull request adds a new WebGPU example demonstrating contact shadows.

Example: https://rawcdn.githack.com/querielo/three.js/kirill/webgpu-contact-shadow/examples/webgpu_shadow_contact.html

[Copilot]

Pull Request Overview

This pull request adds a new WebGPU example demonstrating contact shadows with Gaussian blur for smooth shadow rendering. The example port from the existing WebGL contact shadows example to use WebGPU's node-based material system with TSL (Three.js Shading Language).

Key Changes

• Implemented contact shadows using WebGPU's node material system and TSL's gaussianBlur function
• Uses uniform nodes for dynamic control of shadow parameters (blur, darkness, opacity)
• Added Inspector-based GUI for real-time parameter adjustment

Reviewed Changes

Copilot reviewed 3 out of 4 changed files in this pull request and generated 2 comments.

File	Description
examples/webgpu_shadow_contact.html	New WebGPU example implementing contact shadows with Gaussian blur using node materials and TSL
examples/files.json	Added new example to the files list in alphabetical order
examples/tags.json	Tagged the example with "shadow", "soft", and "tsl" for categorization
examples/screenshots/webgpu_shadow_contact.jpg	Screenshot for the example thumbnail

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