Refactor code organization

Is your feature request related to a problem? Please describe.
Re-organize binding codes according to Skia header structure. Also rename stub class name in a consistent manner; e.g., from:

py::class_<SkColorFilter, sk_sp<SkColorFilter>, SkFlattenable> colorfilter(m, "ColorFilter");

to:

py::class_<SkColorFilter, sk_sp<SkColorFilter>, SkFlattenable> ColorFilter(m, "ColorFilter");

This is to improve code maintenance over skia version upgrades.

Describe the solution you'd like
In Skia, header files are organized under different directories by categories, such as include/core/*.h or include/gpu/*.h. Presumably, the python package should be also split according to this structure.

Describe alternatives you've considered
n/a

Additional context
n/a

首先，将现有的绑定代码与 Skia 的 include/core/.h 和 include/gpu/.h 头文件类别进行映射。检查当前的 stub 类命名，包括 ColorFilter 示例，并在修改之前确定包布局和命名范围。绑定遵循头文件结构且 stub 类名称保持一致，即表示完成。