Add dispose function to VREffect. Otherwise it causes a memory leak if it is instantiated several times -> always add eventlistener on window (#9787)

VREffects/VRControls: Reverted internal vrdisplays removal so it doesn't break backward compatibility.

* added listDisplays

* updated vreffect and cubes example

* updated controls

* merged getDisplays with listDisplays

* switched to setDisplay/getDisplay

* updated example

* VREffect: Check VRLayer configuration on every render

* mrdoobapproves: spacing fixes #9778

* adding the lerp to Math class #9728

Adding the precise  interpolation from point x to y using factor t.

* Adding the documentation for Lerp function

Adding the lerp in doc support

* Documentation: Add more live examples

* Documentation: Correct format