- RadioButtonGroup values for name/required override those of RadioButton

- Fix reactivity when RadioButton is used without group
- Use `readable` when writing is not required
- Use `readonly` to make intent clearer.
- Remove default values for `required` and `name`
  - More consistent with other input types
  - Means that `name` attribute will not be added for no reason

types/RadioButtonGroup/RadioButtonGroup.svelte.d.ts

@@ -18,13 +18,13 @@ export interface RadioButtonGroupProps extends RestProps {
 
   /**
    * Set to `true` to require the selection of a radio button
-   * @default false
+   * @default undefined
    */
   required?: boolean;
 
   /**
    * Specify a name attribute for the radio button inputs
-   * @default ""
+   * @default undefined
    */
   name?: string;