Adding advanced WebXR hands examples

[felixtrz]

Adding 3 advanced WebXR hands examples:

Button Press Example - Using collision detection to trigger events when user press down button
Point-and-Click Example - Using a droplet-shaped target ray to point and click on buttons in the scene
Point-and-Drag Example - Using a droplet-shaped target ray to drag and drop objects in the scene

[mrdoob]

Looking good!

[mrdoob]

Instead of adding a custom ECS, what do you think about using ECSY instead?

[felixtrz]

Instead of adding a custom ECS, what do you think about using ECSY instead?

Great point! Let me update the examples to using ECSY.

[felixtrz]

Updated the new hands examples with ECSY implementation @mrdoob

[felixtrz]

changes merged in #21712