Support "true portable" builds

[pfirsich]

"Portable Builds" here refers how it is used in the butler documentation: https://itch.io/docs/butler/single-files.html

Regular windows builds will put all the game files (code + assets) in the .love file and fuse it to the .exe. This might be not ideal for butler to figure out diffs. An alternative way would be to do love.filesystem.mount(love.filesystem.getSourceBaseDirectory()) if the game is fused, only put the code in the .love file and the assets next to the final, fused .exe.

First of all it has to be tested out if the diffs are significantly larger and butler really does have trouble figuring it out.