Add watchOS as a supported destination

[tatsuky]

Hi, I'm new to this project. This PR adds the watchOS support for the supportedDestinations configuration, initially added in #1336.

Changes

• Added the watchOS case to the SupportedDestination enum.
• Added a configuration template for watchOS.
• Added test cases.

Tests

You can use the following example spec to verify the changes.

name: MyProject
options:
  bundleIdPrefix: com.myapp
targets:
  MyWatchKitApp:
    type: application
    platform: watchOS
    sources: [MyWatchKitApp]
    settings:
      MARKETING_VERSION: 1
      CURRENT_PROJECT_VERSION: 1
    dependencies:
      - target: MyFramework
    info:
      path: MyWatchKitApp/Info.plist
      properties:
        WKCompanionAppBundleIdentifier: com.myapp
        WKApplication: true
  MyFramework:
    type: framework
    supportedDestinations: [iOS, watchOS]
    platform: iOS
    sources: [MyFramework]
    settings:
      GENERATE_INFOPLIST_FILE: YES

Thanks.