Improve Windows support for create-mainframe

[andreterron]

create-mainframe uses zx to run scripts, which relies on bash.

Running npm create mainframe on my Windows machine resulted in an error:

/bin/bash: npm: command not found

Options

1. Use an alternative method to run scripts
2. Update create-mainframe to download a release instead of cloning the repo

While (1) should be fairly straightforward, it's just a patch. create-mainframe clones the repo, and that isn't the best way to distribute Mainframe.