Proxy interceptors and strict-mode write errors match the standard

Two more corners of the language brought in line with the standard. Programs that use JavaScript's Proxy feature to intercept how an object is inspected or changed now have those interceptors honored for every standard operation, not just some, so the reactive-data and virtualization libraries built on it behave correctly. And in strict mode, an assignment the standard says cannot succeed, writing to a read-only or frozen property or adding one to a sealed object, now reports a clear error instead of failing silently, exactly as other browsers do. Ordinary code that was already correct is unchanged. The JavaScript language aspect moves 92 to 93; the overall platform score stays at 70.