Consolidate editor/canvas resize handles

[richtabor]

While writing up #60052, I noticed that we have two different resize handle behaviors for manipulating the size of the editor (either from the Site Editor canvas, or from isolated/focus views).

I'd expect them to be the same, referencing the resize handle display and behavior from the Site Editor canvas (the one that grows and turns blueberry when interacted with).

Visual

resize-handles.mp4