Draggable divider in resizable PageLayout component should be more obvious

[dylanatsmith]

We got feedback that it’s not clear the PageLayout panes can be resized, and it’s not clear where you would drag to resize. See https://github.com/github/codespaces/issues/19538

I’m proposing a simple solution like adding a grabber icon atop the divider as an affordance:

Since the divider already has good visual hover and active states, maybe the icon is hidden in those states so we don’t need to handle them.

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[mperrotti]

We discussed this in today's Primer patterns working session and the designers on the call liked @dylanatsmith 's proposal to add a grabber icon atop the divider as an affordance.