Add fourslash test for contextually-typed this

Regression test for #10972

tests/cases/fourslash/memberListOnContextualThis.ts

@@ -0,0 +1,12 @@
+/// <reference path='fourslash.ts'/>
+////interface A {
+////    a: string;
+////}
+////declare function ctx(callback: (this: A) => string): string;
+////ctx(function () { return th/*1*/is./*2*/a });
+
+goTo.marker('1');
+verify.quickInfoIs("this: A");
+goTo.marker('2');
+verify.memberListContains('a', '(property) A.a: string');
+