Implement custom animations

[bluecarrot16]

Sanderfrenken's generator recently implemented a feature where certain assets can declare new animations by re-arranging existing animation frames. Example: the fishing rod uses frames from the "thrust" animation, played in a different order. https://sanderfrenken.github.io/Universal-LPC-Spritesheet-Character-Generator/#?body=Body_color_light&head=Human_male_light&weapon=Rod_rod .

Implementation there: https://github.com/sanderfrenken/Universal-LPC-Spritesheet-Character-Generator/blob/master/sources/chargen.js#L406-L471

Implementation here would likely be related to #2