[split-layout] minimal variant

[jouni]

Describe your motivation

Add a variant that hides the splitter completely (no visible divider). When hovered (or focused with the keyboard, once implemented), the splitter turns visible, overlaid on top of the content.

See Slack for inspiration:

Screen.Recording.2026-03-11.at.10.54.52.mov

Describe the solution you'd like

<vaadin-split-layout theme="minimal">

Describe alternatives you've considered

No response

Additional context

No response