Add string literal completion test

Author: timsuchanek

tests/cases/fourslash/completionsWithGenericStringLiteral.ts

@@ -0,0 +1,11 @@
+/// <reference path="fourslash.ts" />
+// @strict: true
+
+//// declare function get<T, K extends keyof T>(obj: T, key: K): T[K];
+//// get({ hello: 123, world: 456 }, "/**/");
+
+verify.completions({
+  marker: "",
+  includes: ['hello', 'world']
+});
+