What?

This PR:

• uses the (new) placement prop instead of the (legacy) position prop for the Popover component in the Block Toolbar
• moves the placement prop from being assigned to the Popover component inline, to the useBlockToolbarPopoverProps prop

Why?

• placement is the prop that should be used for positioning the Popover component. The legacy position prop will soon be deprecated
• it makes sense for the prop to be part of the return value of useBlockToolbarPopoverProps, because the logic inside that hook really only makes sense with top/bottom placements. And so, having placement closer to the useBlockToolbarPopoverProps logic made more sense to me (this was also reported in Ensure block toolbar doesn't overlap block by modifying forcePosition and shift popover props #42887 (comment))

How?

Since placement needs to be always returned (regardless of whether there is enough space above the block or not), I've created a COMMON_PROPS object that is spread into both DEFAULT_PROPS and RESTRICTED_HEIGHT_PROPS

Testing Instructions

Play around with the block toolbar, make sure that its placement is the same as on trunk:

• both in the post editor and in the site editor
• both for blocks next to the very top of the page, and for block further down the page