fix: add EOL for files

Author: razonyang

packages/decap-cms-core/src/backend.ts

@@ -1313,7 +1313,12 @@ export class Backend {
     const format = resolveFormat(collection, entry.toJS());
     const fieldsOrder = this.fieldsOrder(collection, entry);
     const fieldsComments = selectFieldsComments(collection, entry);
-    return format && format.toFile(entry.get('data').toJS(), fieldsOrder, fieldsComments);
+    let content = format.toFile(entry.get('data').toJS(), fieldsOrder, fieldsComments);
+    if (content.slice(-1) != '\n') {
+      // add the EOL if it does not exist.
+      content += '\n';
+    }
+    return content;
   }
 
   fieldsOrder(collection: Collection, entry: EntryMap) {