Custom element example

[BillyDM]

Create an example that showcases how to construct a simple custom element. The custom element should render at least one quad and one mesh primitive. It should respond to mouse and keyboard input in some way.

[ottergal]

Is this being worked on? If not, I have some commented "template code" for a "minimum viable element" that I think I could turn into an example element with fairly minimal effort.

[BillyDM]

Ah yeah, I should probably mention in the readme/project board that I'm currently reworking the API of Yarrow to make it more data-driven. I'll post an update when the new API is ready.