Support all three official Unity Render pipelines

[happy-turtle]

This package should work with all official render pipelines:

• [x] Built-in Render Pipeline
• [x] High Definition Render Pipeline
• [x] Universal Render Pipeline

I tried this already but always failed with this question: How is it possible for three versions to live in one package and conditionally compile for a certain render pipeline? Or do we need three different package versions?

[happy-turtle]

The completion of #9 will hopefully make this a whole lot easier. Using assembly definitions should make it possible to create package modules that conditionally compile for each render pipeline.

• https://docs.unity3d.com/Manual/cus-asmdef.html
• https://docs.unity3d.com/Manual/ScriptCompilationAssemblyDefinitionFiles.html

[happy-turtle]

Work is in progress #14

[happy-turtle]

Merged #14