feat: Harmonise filename setting with canvas-sketch settings

[delucis]

canvas-sketch exposes file, name, prefix, and suffix settings that are used to determine naming for exported files, while pellicola currently uses only a filename setting.

Additionally, pellicola defaults to saving videos as out.mp4 where canvas-sketch uses a timestamp, which might be a more helpful default.

Separating settings like canvas-sketch does will also allow decoupling of file naming from which extension should be used.