Tracks some follow-ups on #82604
Most of the suggestions were implemented in #82526 already (we can re-use those commits)

// PROTOTYPE consider using a separate classification type for unions so users can color them differently
// PROTOTYPE consider having a separate NavigateToItemKind category for unions
PROTOTYPE Consider displaying unions as union MyUnioninstead ofstruct MyUnion and with a separate classification type
PROTOTYPE Consider listing case types in QuickInfo tooltip for unions

Note: this may require new compiler APIs, such as IsUnion to INamedTypeSymbol

Relates to test plan #81074