feat: shareable preset configs (#658)

This PR implements eslint-inspired shareable configs. With this feature, we:
- Add new config option "extends"
  * This is an array of preset config names that the current config or preset extends/inherits from
- Add new config option "description"
  * Each preset config should include a description of its functionality that is human-readable
- Provide a set of commonly use configs called "preset defaults"
- Provide two "full" preset configs: "app" and "library"
   * Almost the same but app pins *all* version numbers while library only pins devdependencies
- Replace regular config in onboarding with one of these two presets
- Generate human-readable description of config in Onboarding PR
- Support npm for hosting configs (both scoped and non-scoped)

Closes #657, Closes #671, Closes #647