Euler has "isEuler", Vector3 has "isVector3"... etc. etc.  Quaternion does not.

Previous implementation relied on average quaternions (t=0.5) for small angles.
Now uses the same approach as Quaternion.slerpFlat (threshold and lerp)

Fix parameters of Quaternion.setFromEuler and Matrix4.makeRotationFromEuler, throw on old parameters use.

This reverts commit 563060ae.

This reverts commit eeae90e1.

* move is* properties to prototypes and remove unused ones

* rebuild

* merge dev -> less-is

* remove rebuild from PR

* convert to ES modules

* rebuild

* move shaders back into glsl files

* reinstate polyfills

* tighten up Vector4.

* tighten up Vector3.

* tighten up Vector2.

* add missing "return this" to Ray3.lookAt().

* tighten up Quaternion.

* missing "return this" on Spherical.

* missing "return this" on Euler.  Tighting up code.