Use bool instead of boolean return type in isVoid builtin (AssemblyScript#1745)

Author: yjhmelody

NOTICE

@@ -35,6 +35,7 @@ under the licensing terms detailed in LICENSE:
 * Chance Snow <git@chancesnow.me>
 * Peter Salomonsen <petersalomonsen@runbox.com>
 * ookangzheng <git-ed@runbox.no>
+* yjhmelody <yjh465402634@gmail.com>
 
 Portions of this software are derived from third-party works licensed under
 the following terms:

std/assembly/builtins.ts

@@ -54,7 +54,7 @@ export declare function isManaged<T>(value?: T): bool;
 
 // @ts-ignore: decorator
 @builtin
-export declare function isVoid<T>(): boolean;
+export declare function isVoid<T>(): bool;
 
 // @ts-ignore
 @builtin