split normal_fragment to normal_fragment_begin and normal_fragment_maps

[sunag]

Follow the same logic of this PR #13277

The problem happen when I use the properties normalMap and/or bumpMap with other code but needing the normal core code.

normal_fragment.glsl -> normal_fragment_begin.glsl + normal_fragment_maps.glsl

[sunag]

/cc @WestLangley @bhouston

[WestLangley]

This looks OK to me.

@mrdoob As before, it is important that this change be listed in the migration notes.

[sunag]

Apparently they are the latest split changes. I test with others properties names and work done.

material.alphaMap =
material.aoMap = 
material.aoMapIntensity = 
material.bumpMap = 
material.bumpScale = 
material.emissiveMap = 
material.color =
material.displacementMap = 
material.displacementScale =
material.displacementBias = 
material.emissive = 
material.emissiveMap = 
material.emissiveIntensity = 
material.envMap = 
material.envMapIntensity =
material.lightMap = 
material.lightMapIntensity = 
material.map = 
material.metalness =
material.metalnessMap =
material.normalMap = 
material.normalScale =
material.refractionRatio = 
material.roughness =
material.roughnessMap = 
material.gradientMap = map;

[mrdoob]

Thanks!

[mrdoob]

@WestLangley Done!