Add missing declaration of Map#get in std/assembly (AssemblyScript#223)

Author: LiaoPeng

NOTICE

@@ -8,6 +8,7 @@ under the licensing terms detailed in LICENSE:
 * Igor Sbitnev <PinkaminaDianePie@gmail.com>
 * Norton Wang <me@nortonwang.com>
 * Alan Pierce <alangpierce@gmail.com>
+* Palmer <pengliao@live.cn>
 
 Portions of this software are derived from third-party works licensed under
 the following terms:

std/assembly/index.d.ts

@@ -524,6 +524,7 @@ declare class Map<K,V> {
   readonly size: i32;
   has(key: K): bool;
   set(key: K, value: V): void;
+  get(key: K): V;
   delete(key: K): bool;
   clear(): void;
 }