Print production warning when using serve

[agubler]

Enhancement

The serve option is not intended to be used to serve applications in production. We should add a warning message when running serve for the user.

[devpaul]

Right now serve fails when not run in a one of the test modes.

Are you thinking of something like this for a warning?

devpaul@6443e0a

It'll warn if NODE_ENV === 'production' or if they (somehow) run serve when not in a test mode,

[matt-gadd]

@devpaul no, the serve option should just consistently warn saying that it's not a production server (regardless of mode). preferably here: https://github.com/dojo/cli-build-app/blob/master/src/main.ts#L81, which uses the logger.