feat: add test for infix class expression after constructor call

Add a new test file `test/constructor/infix_class_expression.wren` that verifies
the ability to use an infix operator on a freshly constructed object instance.
The test creates a `Foo` class with a `+` method and checks that `new Foo + "value"`
correctly evaluates to `"Foo value"`. Also includes a TODO comment noting other
constructor expression patterns that should be tested.

test/constructor/infix_class_expression.wren

@@ -0,0 +1,7 @@
+class Foo {
+  + other { return "Foo " + other }
+}
+
+io.write(new Foo + "value") // expect: Foo value
+
+// TODO: Other expressions following a constructor, like new Foo.bar("arg").