Custom user chunks to customize lit / standard shaders

[mvaligursky]

• removed private and non-functional StandardMaterial.customFragmentShader
• added custom empty chunks allowing vertex and fragment shader of StandardMaterial and LitMaterial to be customized:

    litUserDeclarationPS: '',  // empty chunk allowing user to add custom code
    litUserDeclarationVS: '',  // empty chunk allowing user to add custom code
    litUserCodePS: '',  // empty chunk allowing user to add custom code
    litUserCodeVS: '',  // empty chunk allowing user to add custom code
    litUserMainStartPS: '',  // empty chunk allowing user to add custom code
    litUserMainStartVS: '',  // empty chunk allowing user to add custom code
    litUserMainEndPS: '',  // empty chunk allowing user to add custom code
    litUserMainEndVS: '',  // empty chunk allowing user to add custom code

These are included into existing shader chunks, either at the very start of the shader, just before the main function, and also at the start and end of the main function, which should cover most common places to inject code (apart from replacing existing chunks)

• also created new chunk litMainPS (similar to existing litMainVS) which is the single entry point to fragment shader for all passes, removing any hardcoded parts. As long as the user knows this is the entry point, they can follow the rest of the shader from there:

[Maksims]

This is amazing, as it will allow an easy injections into shaders!