Introduce terminal/sys.zig which provides runtime-swappable function
pointers for operations that depend on external implementations. This
allows embedders of the terminal package to swap out implementations
at startup without hard dependencies on specific libraries.
The first function exposed is decode_png, which defaults to a wuffs
implementation. The kitty graphics image loader now calls through
sys.decode_png instead of importing wuffs directly.
This allows us to enable Kitty graphics support in libghostty-vt
for all targets except wasm32-freestanding.
