Use TypeScript check for the Accordion's defaults immutability

`Object.freeze` only does a shallow freeze of the object, meaning we have nothing checking our code
against something setting one of the keys of `i18n`.

The `ReadonlyDeep` type from `type-fest` allows TypeScript to check that even deep properties cannot be updated by our code.

package.json

@@ -90,6 +90,7 @@
     "stylelint": "^16.9.0",
     "stylelint-config-gds": "^2.0.0",
     "stylelint-order": "^6.0.4",
+    "type-fest": "^4.26.1",
     "typescript": "^5.5.4"
   },
   "optionalDependencies": {

packages/govuk-frontend/src/govuk/components/accordion/accordion.mjs

@@ -577,9 +577,9 @@ export class Accordion extends GOVUKFrontendComponentConfigurable {
    *
    * @see {@link AccordionConfig}
    * @constant
-   * @type {AccordionConfig}
+   * @type {ReadonlyDeep<AccordionConfig>}
    */
-  static defaults = Object.freeze({
+  static defaults = {
     i18n: {
       hideAllSections: 'Hide all sections',
       hideSection: 'Hide',
@@ -589,7 +589,7 @@ export class Accordion extends GOVUKFrontendComponentConfigurable {
       showSectionAriaLabel: 'Show this section'
     },
     rememberExpanded: true
-  })
+  }
 
   /**
    * Accordion config schema
@@ -605,6 +605,11 @@ export class Accordion extends GOVUKFrontendComponentConfigurable {
   })
 }
 
+Accordion.defaults.rememberExpanded = false
+if (Accordion.defaults.i18n?.hideSection) {
+  Accordion.defaults.i18n.hideSection = 'Some new string'
+}
+
 /**
  * Accordion config
  *
@@ -641,3 +646,8 @@ export class Accordion extends GOVUKFrontendComponentConfigurable {
 /**
  * @typedef {import('../../common/configuration.mjs').Schema} Schema
  */
+
+/**
+ * @template T
+ * @typedef {import('type-fest').ReadonlyDeep<T>} ReadonlyDeep<T>
+ */