feat: add a method for subclasses of FieldVariable to get the default type. (#8453)

core/field_variable.ts

@@ -322,6 +322,15 @@ export class FieldVariable extends FieldDropdown {
     return this.variable;
   }
 
+  /**
+   * Gets the type of this field's default variable.
+   *
+   * @returns The default type for this variable field.
+   */
+  protected getDefaultType(): string {
+    return this.defaultType;
+  }
+
   /**
    * Gets the validation function for this field, or null if not set.
    * Returns null if the variable is not set, because validators should not