Building should not require an Internet connection (`npm run fetch` is a problem)

Expected Behavior

Once the repository is cloned, it should be possible to make development builds of the application repeatedly without an Internet connection.

Note that some signed builds will always require an Internet connection, as will Apple-notarized builds.

Actual Behavior

One of the build steps downloads our asset libraries (sprites, costumes, and sounds) from the Scratch asset servers. This causes trouble when trying to build without an Internet connection, or when building in a network which blocks connections to the Scratch asset servers.

This causes occasional issue reports such as #140 or #69.

This also introduces an extra dependency on the Scratch infrastructure in the build process, which isn't ideal for forks. It's not particularly difficult to change this dependency, but it should probably be independent by default.

Start with the npm run fetch step and trace where the build downloads the sprites, costumes, and sounds from Scratch asset servers. Check how development builds source these asset libraries and determine what must change so repeated builds work offline while signed and Apple-notarized builds may still use the Internet. Done means a cloned repository can make development builds without contacting those servers.