LibJS: Create const variables in ForIn/OfBodyEvaluation in strict mode

Our implementation of environment.CreateImmutableBinding(name, true)
in this AO was not correctly initializing const variables in strict
mode. This would mean that constant declarations in for loop bodies
would not throw if they were modified.

To fix this, add a new parameter to CreateVariable to set strict mode.
Also remove the vm.is_strict mode check here, as it doesn't look like
anywhere in the spec will change strict mode depending on whether the
script itself is running in script mode or not.

This fixes two of our test-js tests, no change to test262.

Userland/Libraries/LibJS/Tests/loops/for-in-basic.js

@@ -99,7 +99,7 @@ describe("special left hand sides", () => {
         }).toThrowWithMessage(ReferenceError, "Invalid left-hand side in assignment");
     });
 
-    test.xfail("Cannot change constant declaration in body", () => {
+    test("Cannot change constant declaration in body", () => {
         const vals = [];
         for (const v in [1, 2]) {
             expect(() => v++).toThrowWithMessage(TypeError, "Invalid assignment to const variable");