fix: update unRegionalGroup type to include string literals instead of generic strings

Author: Tade Strehk

index.d.ts

@@ -34,7 +34,7 @@ export interface Country {
   independent: boolean
   status: string
   unMember: boolean
-  unRegionalGroup: string
+  unRegionalGroup: "African Group" | "Asia and the Pacific Group" | "Eastern European Group" | "Latin American and Caribbean Group" | "Western European and Others Group" | ""
   currencies: { [currencyCode: string]: Currency }
   idd: IntlDirectDialingCode
   capital: string[]