Implement Size destructuring

[olivervogel]

By decoupling Size from Polygon.

[$width, $height] = $image->size();
['width' => $width, 'height' => $height] = $image->size();

Closes #1480

[Abdooo2235]

Thanks for picking this up — the shape is exactly what I had in mind, and the keyed-destructuring is a nice addition.

One small thing on Size::offsetGet: the default branch throws \ErrorException. That's conventionally reserved for error-handler-converted PHP errors. For "unknown offset on a read-only ArrayAccess object," PHP's stdlib convention is \OutOfBoundsException (or \InvalidArgumentException — which is also what Size's constructor uses for negative width/height in this codebase). Happy to send a small PR against your branch if useful.

Either way, looking forward to seeing this land.

[olivervogel]

I see what you mean. I don't think InvalidArgumentException is quite the right choice here, since this isn't a function argument in the traditional sense and the exception is for semantically for bad function/method arguments.

I replaced it with RuntimeException, which matches the other ArrayAccess methods and keeps it in the libraries exception hierarchy.