Expose FirebaseCore via SPM (#13659)

Author: paulb777

Package.swift

@@ -69,6 +69,10 @@ let package = Package(
       name: "FirebaseStorageCombine-Community",
       targets: ["FirebaseStorageCombineSwift"]
     ),
+    .library(
+      name: "FirebaseCore",
+      targets: ["FirebaseCore"]
+    ),
     .library(
       name: "FirebaseCrashlytics",
       targets: ["FirebaseCrashlytics"]