[unity Editor Extension] the pre-processing and post-processing pictures of unity assets are automatically transferred to sprite2d

Here is mainly the use of Unity Resource pre-processing and post-processing API：AssetPostprocessor
Refer to the official introduction ：https://docs.unity.cn/ScriptReference/AssetPostprocessor.html

AssetPostprocessor Allows you to connect to the import pipeline and run scripts before or after importing assets .
During model import , Functions are called in the following order ：
1.OnPreprocessModel Called at the beginning, you can override the ModelImporter Set up .

2. After importing meshes and materials , Create a GameObject hierarchy from the imported nodes . Use OnPostprocessMeshHierarchy Change hierarchy . Each represents the import node GameObject Are endowed with corresponding MeshFilter、MeshRenderer and MeshCollider Components . Assigning materials to MeshRenderer Before , Would call OnAssignMaterialModel function .

3. stay GameObject initialization MeshRenderers And it exists “userdata” after OnPostprocessGameObjectWithUserProperties Called . This occurs before child GameObjects are generated .

4. If animation generation was not disabled in the previous phase （ see also ModelImporter.generateAnimations）, It will generate S