GLTFExporter: Add support for 4 UV channels.

[bhouston]

Description

Three.js now supports multiple UV channels per mesh, up to 4:

three.js/src/renderers/webgl/WebGLPrograms.js

Lines 160 to 162 in 76e1fb1

	const HAS_ATTRIBUTE_UV1 = !! geometry.attributes.uv1;
	const HAS_ATTRIBUTE_UV2 = !! geometry.attributes.uv2;
	const HAS_ATTRIBUTE_UV3 = !! geometry.attributes.uv3;

three.js/src/renderers/webgl/WebGLProgram.js

Lines 585 to 587 in 76e1fb1

	parameters.vertexUv1s ? '#define USE_UV1' : '',
	parameters.vertexUv2s ? '#define USE_UV2' : '',
	parameters.vertexUv3s ? '#define USE_UV3' : '',

The GLTFImporter supports 4 channels per mesh of UVs:

https://github.com/mrdoob/three.js/blob/dev/examples/jsm/loaders/GLTFLoader.js#L2116-L2118

But the GLTFExporter only support 2 channels per mesh:

three.js/examples/jsm/exporters/GLTFExporter.js

Lines 1647 to 1650 in 76e1fb1

	uv: 'TEXCOORD_0',
	uv1: 'TEXCOORD_1',
	color: 'COLOR_0',
	skinWeight: 'WEIGHTS_0',

This PR fixes that discrepancy.

This contribution is funded by Threekit