Need an example of a simple material extension as a vertex shader

## What problem does this solve or what need does it fill?

I would like a nice simple example of an ExtensionMaterial that uses a custom wgsl shader of just a vertex shader, perhaps that accepts a prop for Velocity and a gradient texture .

In this way , the vertices (mapped by uv map) will be translated by the (velocity x gradient texture)

For simulating grass, hair, etc

## What alternative(s) have you considered?

Other solutions to solve and/or work around the problem presented.

## Additional context

Any other information you would like to add such as related previous work,
screenshots, benchmarks, etc.

Start by locating Bevy's ExtensionMaterial examples and the existing WGSL material and vertex-shader entry points. Add a simple example that accepts velocity and a gradient texture, applies their combination to vertex translation using UVs, and demonstrates the grass or hair use case.

Written by the indexing model from the issue text.