fix: standard core index not includes string namespace

ark120202 · ark120202 · commit c829ded1bfd5 · 2019-02-24T20:23:38.000+05:00
diff --git a/core/index.d.ts b/core/index.d.ts
@@ -6,4 +6,5 @@ import './math';
 import './metatable';
 import './modules';
 import './os';
+import './string';
 import './table';
