Provide a particle shader in extras

[TokisanGames]

Description

Take Tom's particle shader found in this commit and discord thread.
tcoxon@2f1c76d
https://discord.com/channels/691957978680786944/1219611622595891271

Get it working with get_height_maps_rid(), and provide a working example for storage in our /extras folder.
Ensure it's existence doesn't conflict with the rest of the addon or demo. Probably no shader_global, make it local. You could provide another version of demo.tscn with it enabled.