Tab Navigation: Set selected and focussed tabs on changes

[RasmusKjeldgaard]

This PR closes #3420

What is the new behavior?

If slotted tab-navigation-items change, we update selected and focussed tabs accordingly. This allows users to set new values for selectedIndex and focusIndex when adding or removing items, so the template and logic stays in sync.

Does this PR introduce a breaking change?

• [ ] Yes
• [x] No

Checklist:

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