feat: add getContainerSize

[livwvil]

Hi! I am building a docking manager on top of this library and have run into a problem. I need to know the container size when Allotment onChange invoked. Actually I need the width, height x and y of each panel inside the Allotment, but this is enough for me to find all the parameters myself. If you want, I can add an advanced story next to VS Code with a docking manager.