Add #Preview and @Test traits for overriding dependencies

[stephencelis]

This PR introduces new traits for overriding dependencies.

In previews:

#Preview(
  .dependencies {
    $0.apiClient = .failsOnSignUp
    $0.date.now = Date(timeIntervalSince1970: 1234567890)
    $0.continuousClock = .immediate
  }
) {
  // The 'apiClient', 'date' and 'continuousClock' dependencies
  // are all overridden in this preview.
  SignUpView(
    model: SignUpModel()
  )
}

In tests:

import DependenciesTestSupport

@Test(
  .dependency(\.apiClient, .failsOnSignUp),
  .dependency(\.date.now, Date(timeIntervalSince1970: 1234567890)),
  .dependency(\.continuousClock, .immediate)  
)
func feature() {
  let model = SignUpModel()
  // The 'apiClient', 'date' and 'continuousClock' dependencies
  // are all overridden in this scope.
}