Improve /posteffect: Add fade-in/out and duration timers

The new /posteffect command is a huge step forward for shader pipelines, but right now it is strictly binary. We can only toggle an effect instantly via add and remove.

When working with heavy fullscreen passes (distortion, chromatic aberration, or blurs), instant cuts feel very jarring. Smooth transitions like recovering from a concussion, flashbang falloff, or entering a cold biome are basically impossible because the pipeline gets killed immediately upon removal.

It would be awesome if we could get transition controls similar to /title times:
/posteffect add <targets> <effect> [fadeIn] [duration] [fadeOut]
The engine could pass an interpolated Progress float (0.0 to 1.0) directly into the pass uniform block. That way, shaders can sample and smoothly mix their output with minecraft:main instead of popping in abruptly.

Same for /posteffect remove <targets> <effect> [fadeOut] - letting the engine interpolate the progress uniform back to 0.0 before unbinding the pipeline pass would prevent sharp visual snaps and make datapack visual effects look infinitely better.