[Observation] Add support for package properties

Author: phausler

lib/Macros/Sources/ObservationMacros/ObservableMacro.swift

@@ -118,6 +118,7 @@ extension DeclModifierListSyntax {
         case .fileprivate: fallthrough
         case .private: fallthrough
         case .internal: fallthrough
+        case .package: fallthrough
         case .public:
           return false
         default:

test/stdlib/Observation/Observable.swift

@@ -31,6 +31,11 @@ public class PublicContainsWeak {
   public weak var obj: AnyObject? = nil
 }
 
+@Observable
+class ContainsPackage {
+  package var contents: Int = 0
+}
+
 @Observable
 class ContainsUnowned {
   unowned var obj: AnyObject? = nil