Improve pytmx SDL2 Loader with Colorkey Support and Cleaner Typing

[JaskRendix]

PR introduces enhancements to the pytmx SDL2 image loader integration for use with pygame._sdl2. The changes are summarized as follows:

• added colorkey support: the image loader now accepts a colorkey to handle transparency. Supports hex strings (with or without #) and RGB(A) tuples
• improved type hinting: added explicit type annotations using ColorLike, PointLike, and Callable from pytmx.pytmx for improved clarity and compatibility
• refactored dataclasses: used @dataclass from dataclasses instead of mixing with dataclasses.dataclass to streamline code style
• colorkey validation with error handling: ensures invalid colorkeys are flagged with descriptive logging and a ValueError
• simplified imports and cleanup: reorganized imports and removed unused symbols to keep the loader tight and readable