Restructure so that `npm publish` can be used for the Windows and macOS workspaces

[jugglinmike]

The bundle created by these commands should include the shared code as well as the relevant platform-specific code, and each should expose at-driver as an executable entry point with an identical command-line interface:

$ npm pack --workspace packages/macos-at-driver-server
$ npm pack --workspace packages/windows-at-driver-server