Fix compiler “Type 'string' is not assignable to type 'PluralRuleName'”

I18n.pluralRulesMap keys are variable so “string” wasn’t compatible when the compiler thought we were using fixed string literals

Might be worth another look at `Object.keys()` in future with:

* microsoft/TypeScript#45464

src/govuk/i18n.mjs

@@ -211,7 +211,7 @@ I18n.prototype.selectPluralFormUsingFallbackRules = function (count) {
  * regardless of region. There are exceptions, however, (e.g. Portuguese) so
  * this searches by both the full and shortened locale codes, just to be sure.
  *
- * @returns {PluralRuleName | undefined} The name of the pluralisation rule to use (a key for one
+ * @returns {string | undefined} The name of the pluralisation rule to use (a key for one
  *   of the functions in this.pluralRules)
  */
 I18n.prototype.getPluralRulesForLocale = function () {
@@ -262,7 +262,7 @@ I18n.prototype.getPluralRulesForLocale = function () {
  * Spanish: European Portuguese (pt-PT), Italian (it), Spanish (es)
  * Welsh: Welsh (cy)
  *
- * @type {Object<PluralRuleName, string[]>}
+ * @type {Object<string, string[]>}
  */
 I18n.pluralRulesMap = {
   arabic: ['ar'],
@@ -348,12 +348,6 @@ I18n.pluralRules = {
   }
 }
 
-/**
- * Supported languages for plural rules
- *
- * @typedef {'arabic' | 'chinese' | 'french' | 'german' | 'irish' | 'russian' | 'scottish' | 'spanish' | 'welsh'} PluralRuleName
- */
-
 /**
  * Plural rule category mnemonic tags
  *