[Typescript] allow passing readonly array to LocalesPlugin (#5646)

Co-authored-by: Reid Barber <reid@reidbarber.com>

Author: stefanprobst

packages/dev/optimize-locales-plugin/LocalesPlugin.d.ts

@@ -1,7 +1,7 @@
 import {UnpluginInstance} from 'unplugin';
 
 type Options = {
-  locales: string[]
+  locales: readonly string[]
 };
 
 declare const plugin: UnpluginInstance<Options, boolean>;