[Shader] Feature set, Variant collection ,Compiling, Loading

[roeas]

• Treate shader features as a set to handle mutually exclusive keywords. #378
• Use shader collections to guide the compiling and loading of shader variants. #381 #382
• Modify shader files to hot-compile and hot-load variants when editor is running.
  • Block the main process from compiling uber shaders only when the editor regains focus. #377 #421
  • Store shader dependencies and update them appropriately.
    convert to #442

[roeas]

Discussion on feature set and variant collection: #362
Hot load: #241
Unity Loading, Prewarming and Chunk: https://docs.unity3d.com/2023.2/Documentation/Manual/shader-loading.html