Ensure JSDoc plural form other is optional (with caveats)

See: #2921

Whilst `other` is required in English and is used as a fallback, it won’t be in all locales

src/govuk/components/character-count/character-count.unit.test.mjs

@@ -92,13 +92,11 @@ describe('CharacterCount', () => {
             const component = new CharacterCount($div, {
               i18n: {
                 charactersUnderLimit: {
-                  one: 'Different custom text. Count: %{count}',
-                  other: 'Different custom text. Count: %{count}'
+                  one: 'Different custom text. Count: %{count}'
                 }
               }
             })
             expect(component.formatCountMessage(1, 'characters')).toEqual('Custom text. Count: 1')
-            expect(component.formatCountMessage(10, 'characters')).toEqual('Different custom text. Count: 10')
             // Other keys remain untouched
             expect(component.formatCountMessage(-10, 'characters')).toEqual('You have 10 characters too many')
             expect(component.formatCountMessage(0, 'characters')).toEqual('You have 0 characters remaining')

src/govuk/i18n.mjs

@@ -364,7 +364,7 @@ I18n.pluralRules = {
  * translations to a component's constructor
  *
  * @typedef {object} TranslationPluralForms
- * @property {string} other - General plural form
+ * @property {string} [other] - General plural form
  * @property {string} [zero] - Plural form used with 0
  * @property {string} [one] - Plural form used with 1
  * @property {string} [two] - Plural form used with 2