Math interpolants and remainings: Move to es6 classes

[yomotsu]

A part of #19984.
A series of Math classes conversion, Part 3 of 3.

This PR contains:

• [ ] Interpolant (The base class)
  • CubicInterpolant
  • DiscreteInterpolant
  • LinearInterpolant
  • QuaternionLinearInterpolant
• [ ] Interpolant.tests.js [*1]

and

• Color

---

*1:
The test has to be changed as well because 'Interpolant.tests' was assuming Interpolant base class to be a Function (using call to extend the base class).

---

edited: Interpolant base class couldn't be converted. I keep it as it is.

[mrdoob]

Seems like webgl_loader_mmd broke.

[yomotsu]

Aww...
It because MMDLoader also assumes Interporant to be a function.

three.js/examples/js/loaders/MMDLoader.js

Line 1840 in 4c6879a

THREE.Interpolant.call( this, parameterPositions, sampleValues, sampleSize, resultBuffer );

Should we keep the Interporant base class as a function? It may break other 3rd party implementations too.

[Mugen87]

Notice that GLTFCubicSplineInterpolant also derives from Interpolant.

three.js/examples/jsm/loaders/GLTFLoader.js

Lines 1218 to 1222 in 908de67

	function GLTFCubicSplineInterpolant( parameterPositions, sampleValues, sampleSize, resultBuffer ) {
	Interpolant.call( this, parameterPositions, sampleValues, sampleSize, resultBuffer );
	}

Interpolant is class that can't be converted now, see #19934 (comment).

[yomotsu]

Thanks for your notice. I'll revert it for now.

[yomotsu]

let me close this PR.
I will open another PR for Color class.