Add the cposteffect command

[xpple]

This PR adds a command to change the post effect of the game renderer. Examples include minecraft:creeper and minecraft:spider which will apply a funny affect (see minecraft.wiki/w/Spectator#Mob_view). These effects are read from assets/minecraft/post_effect into Minecraft.getInstance().getShaderManager().compilationCache.configs.postChains() but not all are actually useable. It might be possible to deduce which ones are by looking at their configs (PostChainConfig), but I just hardcoded the working ones.