Add support for dynamic sides and targets

[peterp]

We support two sides: api and web in RedwoodJS. This pull-request adds the ability to specify many sides, to provide configuration for those sides, and to have our build/ dev/ test/ lint/ generator tooling support those sides.

The idea is to move us out of a space where we're only supporting an "api" and "web" side, by removing the references to "api" and "web" in the code and making them controlled via configuration.

🕊This code is backwards compatible and will not introduce any breaking changes.

• Make dev-server accept a side param
• Make CLI "sides aware"
• Make core configurations "side aware"

The future idea is that we could add first class support for electron and react-native.

# redwood.toml

[app]
   path = './app/renderer'
   target = 'react-native' # 'react-native-expo'

[desktopMain]
  path = './desktop/main'
  target = 'electron-main'

[desktopRenderer]
  path = './desktop/renderer'
  target = 'electron-renderer'

[web]
   path = './web'
   target = 'browser'

[api]
   path = './api'
   target = 'node'  # 'node-netlify' | 'node-aws' | 'node-zeit' | 'node-express'

[peterp]

A bunch of this code was merged into #573, the concept of sides is not going away - but will be added more incrementally.