Updated 18next exists method and added ability to use string interpolation arguments (#16124)

* Updated i18next extists function definition 

and allow for additional interpolation options to be passed as TranslationOptions

* Updated 18next test

* i18next fixed typing of exists key parameter

* i18next Fixed test typo

types/i18next/i18next-tests.ts

@@ -10,12 +10,14 @@ i18n.init({
     resources: {
         en: {
             translation: {
-                helloWorld: 'Hello, world!'
+                helloWorld: 'Hello, world!',
+                helloWorldInterpolated: 'Hello, {{name}}!'
             }
         },
         ru: {
             translation: {
-                helloWorld: 'Привет, мир!'
+                helloWorld: 'Привет, мир!',
+                helloWorldInterpolated: 'Привет, {{name}}!'
             }
         }
     },
@@ -61,6 +63,14 @@ i18n.t('helloWorld', <i18n.TranslationOptions> {
     count: 10
 });
 
+i18n.t('helloWorldInterpolated', <i18n.TranslationOptions> {
+    defaultValue: 'default',
+    count: 10,
+    name: "world"
+});
+
+i18n.exists("helloWorld");
+
 const options:i18n.Options = i18n.options;
 const currentLanguage:string = i18n.language;
 const userLanguageCodes:string[] = i18n.languages;

types/i18next/index.d.ts

@@ -49,6 +49,8 @@ declare namespace i18n {
         joinArrays?: string;
         postProcess?: string | any[];
         interpolation?: InterpolationOptions;
+        //add an indexer to assure that interpolation arguments can be passed
+        [x: string]: any;
     }
 
     interface Options {
@@ -106,7 +108,7 @@ declare namespace i18n {
 
         t(key: string, options?: TranslationOptions): string | any | Array<any>;
 
-        exists(): boolean;
+        exists(key: string, options?: TranslationOptions): boolean;
 
         setDefaultNamespace(ns: string): void;