FancyZones - span-only zone overlay

### Description of the new feature / enhancement

Let's say I have a layout like this (simplified):

I can easily fit my window into two bottom middle zones just by placing the mouse on the middle border, easy.

But when I want to span the window into four bottom middle zones, I need to drag the mouse across quite a long distance. I don't see any way I can span the window just by positioning the mouse *somewhere* and pressing *something*.

For me it would be great if it was possible to define "span-only zones" which could perhaps be overlaid over the regular zones.
Let's use the initial example with added span-only zone.
When you place the mouse at the yellow cross, only the regular zones would activate.

But when placed at the span-only zone, it would activate all zones below it.

The actual implementation doesn't have to be an overlay, that was just my idea on how this feature could work.

### Scenario when this would be used?

I'm using a big 55" screen divided into many small zones and I'm heavily using the "drag to span" feature. There are some sizes I'm using very often (like normal vs ultrawide), so it would be great to have an easier / faster way to access these sizes.

### Supporting information

_No response_

Start by reviewing FancyZones' existing drag-to-span behavior and how regular zones are activated. Determine how a user-configured span-only zone would be represented and how it would activate the zones beneath it, then verify the requested regular-zone and span-only-zone scenarios.

Written by the indexing model from the issue text.