Reorganize Layout

Rearranged the file/directory layout of the mod to reflect the future development of the game.

The idea was to keep everything that's distinctly related together. For example, weapons.yaml is in the rules folder instead of the root directory because it defines the rules for the game's weapons; the music directory is where the soundtrack will be stored, etc.

From here, the actual building of the mod and the removal of the original content can begin, to build the actual game out of it.