feature(types): add ability to type translation keys

This is a first step towards solving i18next#1504
It allows you to type your translation keys freely. Which allows you to type them using the template literals of Typescript 4.1 for nested keys.

index.d.ts

@@ -2,9 +2,23 @@ export interface FallbackLngObjList {
   [language: string]: string[];
 }
 
-export type FallbackLng = string | string[] | FallbackLngObjList | ((code:string) => string | string[] | FallbackLngObjList);
+/**
+ * Extend this with a 'keys' key where you can add all the available translation keys
+ */
+export interface TranslationKeys {}
+
+export type FallbackLng =
+  | string
+  | string[]
+  | FallbackLngObjList
+  | ((code: string) => string | string[] | FallbackLngObjList);
 
-export type FormatFunction = (value: any, format?: string, lng?: string, options?: InterpolationOptions & { [key: string]: any }) => string;
+export type FormatFunction = (
+  value: any,
+  format?: string,
+  lng?: string,
+  options?: InterpolationOptions & { [key: string]: any },
+) => string;
 
 export interface InterpolationOptions {
   /**
@@ -639,7 +653,7 @@ export type Callback = (error: any, t: TFunction) => void;
  * Uses similar args as the t function and returns true if a key exists.
  */
 export interface ExistsFunction<
-  TKeys extends string = string,
+  TKeys extends TFunctionKeys = Keys,
   TInterpolationMap extends object = StringMap
 > {
   (key: TKeys | TKeys[], options?: TOptions<TInterpolationMap>): boolean;
@@ -650,13 +664,18 @@ export interface WithT {
   t: TFunction;
 }
 
+/**
+ * Gets the defined translation keys if they have been overridden, 'string' type otherwise
+ */
+type Keys = TranslationKeys['keys'] extends string ? TranslationKeys['keys'] : string;
+
 export type TFunctionResult = string | object | Array<string | object> | undefined | null;
 export type TFunctionKeys = string | TemplateStringsArray;
 export interface TFunction {
   // basic usage
   <
     TResult extends TFunctionResult = string,
-    TKeys extends TFunctionKeys = string,
+    TKeys extends TFunctionKeys = Keys,
     TInterpolationMap extends object = StringMap
   >(
     key: TKeys | TKeys[],
@@ -665,7 +684,7 @@ export interface TFunction {
   // overloaded usage
   <
     TResult extends TFunctionResult = string,
-    TKeys extends TFunctionKeys = string,
+    TKeys extends TFunctionKeys = Keys,
     TInterpolationMap extends object = StringMap
   >(
     key: TKeys | TKeys[],