Site Editor: Resizable editor canvas should take window resizes into account

[mirka]

Part of the resizable editor canvas code relies on some width calculations that are only done on first render.

gutenberg/packages/edit-site/src/components/resizable-frame/index.js

Lines 99 to 103 in a2bcf95

	useEffect( () => {
	const { offsetWidth, offsetHeight } = frameRef.current.resizable;
	initialComputedWidthRef.current = offsetWidth;
	initialAspectRatioRef.current = offsetWidth / offsetHeight;
	}, [] );

This can cause the resizing behavior to be glitchy after a user resizes the browser window. There should be a resize listener in place to recalculate the reference widths.