Improve batchConv type checking (#33)

* Improve batchConv type checking

* Update typings.d.ts

Author: sunafterrainwm

typings.d.ts

@@ -20,9 +20,9 @@ declare module 'ext.gadget.HanAssist' {
    * @param locale locale, defaults to `wgUserLanguage`
    * @returns converted candidates dictionary
    */
-  export function batchConv(
-    candidatesDict: Record<string, string | Candidates>, locale?: string,
-  ): Record<string, string>;
+  export function batchConv<T extends string>(
+    candidatesDict: Record<T, string | Candidates>, locale?: string,
+  ): Record<T, string>;
 
   global {
     /** @deprecated Use `HanAssist.conv` instead */