feat(dia.HighlighterView): enable positioning of highlighter based on opt.z

[MartinKanera]

Description

This change introduces the ability to set the stacking order (z-index) for highlighters within the stacking context of dia.CellView. The opt.z controls whether a highlighter is inserted before or after other elements in the DOM hierarchy.

Documentation

Update layer and z sections of the documentation to reflect the changes.

https://docs.jointjs.com/api/dia/HighlighterView/#options

layer

layer - the stacking context of the highlighter. Applicable for mountable highlighters only.

null	Render the highlighter within the dia.CellView, with its position determined by the provided z value. By default, the highlighter is rendered above the cell.
back	Render the highlighter behind all the cells.
front	Render the highlighter in front of all the cells.

z

z - determines the stacking order (z-index) of the highlighter within the specified layer or directly within dia.CellView.

• If layer is defined, the highlighter is inserted into the specified layer at the correct position based on z.
• If layer is not defined, z represents the index within dia.CellView. If z is a valid number and corresponds to an existing child index, the highlighter is inserted before that child (e.g., if z: 0, the highlighter will be inserted before the first child element of the dia.CellView - effectively resulting in prepending it under everything else); otherwise, it is appended as the last child.

Applicable for mountable highlighters only.