feat(types): add generateSpecCacheKey to Fig.Subcommand (#105)

types/index.d.ts

@@ -668,6 +668,13 @@ declare namespace Fig {
      */
     generateSpec?: (tokens: string[], executeCommand: ExecuteCommandFunction) => Promise<Spec>;
 
+    /**
+     * Generating a spec can be expensive, but due to current guarantees they are not cached.
+     * This function generates a cache key which is used to cache the result of generateSpec.
+     * If `undefined` is returned, the cache will not be used.
+     */
+    generateSpecCacheKey?: Function<{ tokens: string[] }, string | undefined> | string;
+
     /**
      * Configure how the autocomplete engine will map the raw tokens to a given completion spec.
      *