Modify return type of DataSnapshot.forEach to boolean | void (#666)

This is to match the Admin SDK:

https://github.com/firebase/firebase-js-sdk/blob/37b98e9271c494a0fb58ca1960f8fcfaec49ade9/packages/database/src/api/DataSnapshot.ts#L137

Co-authored-by: joehan <joehanley@google.com>
Co-authored-by: Lauren Long <laurenzlong@users.noreply.github.com>

Author: 3 people

src/providers/database.ts

@@ -476,7 +476,7 @@ export class DataSnapshot {
    * @return `true` if enumeration was canceled due to your callback
    *   returning `true`.
    */
-  forEach(action: (a: DataSnapshot) => boolean): boolean {
+  forEach(action: (a: DataSnapshot) => boolean | void): boolean {
     const val = this.val();
     if (_.isPlainObject(val)) {
       return _.some(