fix(create-neon): Explicit project type

[dherman]

This PR changes the syntax of create-neon to require the user to explicitly specify the Neon project type, either by passing --lib or --app at the command-line, or otherwise by interactively asking them via stdin.

The reason for this is a limitation of npm init: the only way it will forward an argument like --lib to create-neon is if the user remembers to prefix it with --:

npm init neon -- --lib my-project

Without the extra --, npm silently ignores the --lib, which is a subtle and surprising hazard. This pitfall is easy to stumble into and difficult to debug.

(I built a demonstration package called create-dherman that you can use to test this behavior. Test it out with npm init dherman --lib foobar vs npm init dherman -- --lib foobar, for example.)

With this PR, users still need the --, but if they forget it, they'll be asked the project type interactively.

[kjvalencik]

The templated JSON is a difficult to tell if it always results in valid JSON. In a future PR, it may be cleaner to generate it in JS and call JSON.stringify.