Add 3D physics interpolation demo

[Calinou]

Demonstrate first person shooter and third person shooter cameras.

This salvages @lawnjelly's work on the demo and further refines it.

• This closes Add 3D physics interpolation demo #1220 by superseding it.

Preview

[lawnjelly]

This is great! 🥳

I've added a bunch more comments and made some variable names clearer, hopefully you can copy the gdscript from:
#1220

I also discovered a bug in the SceneTreeFTI for toggling interpolation on / off 😊 , but have made a small fix PR so hopefully that will be closed for 4.5.

[Calinou]

I've added a bunch more comments and made some variable names clearer, hopefully you can copy the gdscript from:
#1220

Done. Note that I've refactored bullet.gd to use built-in Godot functionality whenever possible, which makes the script a fair bit shorter. I've also removed the scale clamping, as current Godot should have no problems with zero-scaled meshes.

[lawnjelly]

Done. Note that I've refactored bullet.gd to use built-in Godot functionality whenever possible, which makes the script a fair bit shorter.

I'm sure that will be fine, the bullet isn't super important.

(I personally don't tend to use or recommend using Timer nodes as I don't find them a good approach to game timing imo, as they aren't accurate for tick based stuff, and can lead to order of operations bugs. But maybe users like them. 😁 )