Add a z-index control for blocks where position is/can be set

[markhowellsmead]

What problem does this address?

Several considerations and many use-cases, for example the stacking order of elements with negative margins (#60347) or the use of multiple sticky blocks.

What is your proposed solution?

Add a control to allow a range of preset z-index options, with a switch for a free number field. (In comparison with existing padding, margin and block gap interface controls).

[markhowellsmead]

See also #32644 and #40464.

[burnuser]

Consider indicating (and maybe editing) the z-index in the List-View for the needed oversight and an efficient management of sticky/overlapping blocks.

[altivero]

I was very excited about creating a pattern with negative margins #60347 until I overlapped a cover with an overlay.

A pattern like this would be nice in the pattern library but doesn't seem possible without additional css or a z-index control.