Fix Array#flat() definition (AssemblyScript#1275)

Author: torch2424

std/assembly/index.d.ts

@@ -1430,7 +1430,7 @@ declare class Array<T> {
   join(separator?: string): string;
   reverse(): T[];
   /** Flattens an array of arrays. If any null entries exist in the array, they are ignored, unlike JavaScript's version of Array#flat(). */
-  flat(): valueof<T>[];
+  flat(): T extends unknown[] ? T : never;
   toString(): string;
 }