feat: import chunk

[homerjam]

This PR includes rather a lot of whitespace and prettier formatting I'm afraid : )

If you like the feature I can tidy it up!

The basic idea is to allow you to easily include chunks from the default threejs shaders in your own, like so:

// myfancyshader.frag

// includes the header chunk
#pragma glslify: import('three/src/renderers/shaders/ShaderLib/meshbasic_vert.glsl.js')(/`(.*)void main/gs)

// custom header

void main() {
  // includes the main chunk
  #pragma glslify: import('three/src/renderers/shaders/ShaderLib/meshbasic_vert.glsl.js')(/void main\(\) {(.*)}/gs)

  // custom main
}