Use app bundles when building for macos

[rparrett]

I think there may be some reason this isn't desirable alluded to here: #13 (comment)

But I've been experimenting with this here, and it seems to generally work.

From what I can see the advantages are:

• Works better with the itch app. Unlike with a .dmg where pressing "launch" just opens up a folder containing the binary.

And the disadvantages:

• Chrome warns you about the file being unusual when downloading

In both cases, actually running the unsigned app (outside of the itch scenario) seems to require the same amount of effort.