Add a custom shader WebGPU example

[mors119]

Summary

Add a more advanced WebGPU example to bracket-terminal.

Problem to solve

There is currently no example showing a custom WebGPU shader or a more advanced WebGPU rendering use case.

This makes it harder to validate the backend and harder for contributors to understand how WebGPU-specific features might be used.

Proposed solution

Add a dedicated WebGPU example that goes beyond the minimal console path and helps validate custom rendering behavior.

Alternatives considered

• Keep only minimal WebGPU examples
• Rely on non-WebGPU examples as reference

Additional context

A dedicated advanced WebGPU example would be useful for both backend validation and contributor guidance.