fix(type): define generate option (#111)

* Fix types for generate option

* fix: restrict `string` values

Co-authored-by: Luke Edwards <[email protected]>

Author: JulienPradet

index.d.ts

@@ -47,8 +47,7 @@ interface Options {
    * By default, the client-side compiler is used. You
    * can also use the server-side rendering compiler
    */
-  // this isn't used yet in plugin
-  // generate?: 'ssr';
+  generate?: 'dom' | 'ssr' | false;
 
   /**
    * Optionally, preprocess components with svelte.preprocess: