[6.x] Add a way to opt out of popper z-index control

[jaygeorge]

Description of the Problem

When a modal, stack, live preview, or fullscreen code editor is open, we pull the Reka popper z-index up so things like Comboboxes and DatePickers inside those overlays still appear on top.

We need to do this because Reka sets z-index: auto; and we can't programmatically override it. So instead we use CSS's :has to test if the DOM has certain things. Not ideal, but there we go.

That blanket rule also elevates poppers that were already open behind an unrelated overlay. For example, a compact array popover can stay open when its “Delete Row” confirmation modal appears — and because both portal to the body, we can't query the DOM structure to understand if the popper belongs to the modal. The popover ends up stacking above the confirmation.

What this PR Does

Adds an opt-out for that z-index rule:

• Popovers and Comboboxes accept excludeManipulationZ
• When set, content gets data-ui-exclude-manipulation-z
• The portal z-index overrides in base.css, stacks.css, and SetPicker skip wrappers that contain that attribute

Consumers that can remain open behind an unrelated overlay can opt out; poppers that belong inside the overlay keep the default elevate behavior.

How to Reproduce

1. Open a Popover or Combobox that remains open when a confirmation modal appears (e.g. compact array (in cp/sites → Custom Attributes → delete a filled row).
2. Without excludeManipulationZ, the popper sits above the confirmation modal.
3. With excludeManipulationZ on that Popover/Combobox, the confirmation modal stacks above the popper as expected.