Accessibility: allow for usage of rem or px in height and width

[angel1199410]

When using height and width in px, there can be issues with accessibility in regards to text-only zoom. Because text zooms and the edges around it do not, edges and nodes rendered with dagree mutate to odd angles and nodes overlap one another.

To recreate the issue:

• Open Firefox and turn on text zoom only mode
• Visit reactflow's example using dagre. Open it in the code sandbox. Change the text in the nodes to be longer for more of an effect.
• Zoom in using browser controls. Note that even though the text gets larger, the edges do not elongate to match.

Allowing for the use of rems (pxValue / 16) will alleviate the text zoom issue and make this library more accessible to users with low vision, or anyone who needs it.