[request] add support for opening .assets and .ress files

[Treehee]

These are also files used by Unity, so it woul dbe quite useful to be able to extract those too.

[nanoNago]

This more or less already works, but you have to know how to load them: use the asset.from_file() function instead of trying to load them from the CLI. The unity2yaml script should actually already handle this.