LibJS: Make eval() return the last value from the executed statement

This is kinda awkward but since the statement we're executing is
actually a JS::Program, we have to get the result via VM::last_value().

Userland/Libraries/LibJS/Tests/eval-basic.js

@@ -0,0 +1,10 @@
+test("basic eval() functionality", () => {
+    expect(eval("1 + 2")).toBe(3);
+
+    function foo(a) {
+        var x = 5;
+        eval("x += a");
+        return x;
+    }
+    expect(foo(7)).toBe(12);
+});