Detect custom build settings have been set at project or target level

If there are new custom build settings, report an error and suggest to declare them via an xcconfig file instead (use the `xcodeproj` Ruby gem to implement that)

Start by locating the Danger plugin entry point that inspects Xcode project or target build settings, then review how the xcodeproj Ruby gem exposes custom settings. Define completion as reporting newly added project- or target-level settings with an xcconfig recommendation, with tests covering both levels.

Written by the indexing model from the issue text.