`Dropdown`: Conduct insight work on RovingFocus and Keyboard interaction

[mrosvik]

Problem

We need to do a proper insight and knowledge-gathering effort around RovingFocus and Keyboard interaction patterns in the Dropdown component.

There are differing opinions on whether Dropdown should support arrow key navigation (with role="menu" / role="menuitem" and roving tabindex), or rely on standard tab navigation. This particularly affects cases like language selectors and other “styled select” patterns.

Todays documentation:
The current short note in the documentation about keyboard behaviour is potentially confusing and may create more work than clarity: https://designsystemet.no/no/components/docs/dropdown/accessibility#tastatur

Goal

Land on a well-considered, evidence-based recommendation for:

• When (if ever) Dropdown should use role="menu" and arrow key navigation
• When standard list semantics and tab navigation are preferable
• Whether RovingFocus should be recommended, optional, or avoided
• Clear guidance for cases like language selectors

The outcome should be a clear and durable recommendation that we can confidently document in Designsystemet.