Update type for Options interface in franc package (#42327)

* Update type for Options interface in franc package

As documented here:

https://github.com/wooorm/franc/issues/80

adding this type enables the use of `franc.all` additionally with an `only` languages array when using TypeScript.

Thank you!

* Update index.d.ts

update with recommended simple type declaration
`ERROR: 16:12  array-type  Array type using 'Array<T>' is forbidden for simple types. Use 'T[]' instead.`

types/franc/index.d.ts

@@ -13,6 +13,7 @@ interface Options {
     minLength?: number;
     whitelist?: ISO6393[];
     blacklist?: ISO6393[];
+    only?: string[];
 }
 
 declare function detect(text: string, options?: Options): ISO6393;