perf: add explicit return type to speed up types

src/defaults/vanilla.ts

@@ -109,7 +109,7 @@ function mergeMaps<Ts extends ReadonlyArray<Readonly<ReadonlyMap<unknown, unknow
 /**
  * Get the last non-undefined value in the given array.
  */
-function mergeOthers<Ts extends ReadonlyArray<unknown>>(values: Ts) {
+function mergeOthers<Ts extends ReadonlyArray<unknown>>(values: Ts): unknown {
   return values.at(-1);
 }