MeshNormalMaterial: rename vertex and fragment shaders

[WestLangley]

Adhering to the three.js nomenclature conventions, this PR renames the MeshNormalMaterial shaders as follows:

normal_vert.glsl.js -> meshnormal_vert.glsl.js

normal_frag.glsl.js -> meshnormal_frag.glsl.js

This is consistent with meshbasic_vert.glsl.js, etc.

It also prevents confusion with normal_vertex.glsl.js and related shader chunks.

[mrdoob]

Thanks!