style: move the function closer to the non-async implementation

Asked by paulb777 [here](firebase#13407 (comment))

Firestore/Swift/Source/AsyncAwait/CollectionReference+AsyncAwait.swift

@@ -42,31 +42,4 @@ public extension CollectionReference {
       }
     }
   }
-
-  /// Encodes an instance of `Encodable` and adds a new document to this collection
-  /// with the encoded data, assigning it a document ID automatically.
-  ///
-  /// See `Firestore.Encoder` for more details about the encoding process.
-  ///
-  /// - Parameters:
-  ///   - value: An instance of `Encodable` to be encoded to a document.
-  ///   - encoder: An encoder instance to use to run the encoding.
-  /// - Throws: `Error` if the backend rejected the write.
-  /// - Returns: A `DocumentReference` pointing to the newly created document.
-  @discardableResult
-  func addDocument<T: Encodable>(from value: T,
-                                 encoder: Firestore.Encoder = Firestore.Encoder()) async throws
-    -> DocumentReference {
-    return try await withCheckedThrowingContinuation { continuation in
-      var document: DocumentReference?
-      document = self.addDocument(from: value, encoder: encoder) { error in
-        if let error {
-          continuation.resume(throwing: error)
-        } else {
-          // Our callbacks guarantee that we either return an error or a document.
-          continuation.resume(returning: document!)
-        }
-      }
-    }
-  }
 }

Firestore/Swift/Source/Codable/CollectionReference+WriteEncodable.swift

@@ -44,4 +44,31 @@ public extension CollectionReference {
     let encoded = try encoder.encode(value)
     return addDocument(data: encoded, completion: completion)
   }
+
+  /// Encodes an instance of `Encodable` and adds a new document to this collection
+  /// with the encoded data, assigning it a document ID automatically.
+  ///
+  /// See `Firestore.Encoder` for more details about the encoding process.
+  ///
+  /// - Parameters:
+  ///   - value: An instance of `Encodable` to be encoded to a document.
+  ///   - encoder: An encoder instance to use to run the encoding.
+  /// - Throws: `Error` if the backend rejected the write.
+  /// - Returns: A `DocumentReference` pointing to the newly created document.
+  @discardableResult
+  func addDocument<T: Encodable>(from value: T,
+                                 encoder: Firestore.Encoder = Firestore.Encoder()) async throws
+    -> DocumentReference {
+    return try await withCheckedThrowingContinuation { continuation in
+      var document: DocumentReference?
+      document = self.addDocument(from: value, encoder: encoder) { error in
+        if let error {
+          continuation.resume(throwing: error)
+        } else {
+          // Our callbacks guarantee that we either return an error or a document.
+          continuation.resume(returning: document!)
+        }
+      }
+    }
+  }
 }