fix: Support two-way binding on Expand in NavMenu and NavTreeView #504
Description
<FluentNavMenuGroup Expanded> will show a menu group expanded by default, and still allow the user to expand/collapse it.
If the FluentNavMenu has to re-render for any reason then this rebinds and causes the menu group to expand again. The way to solve this is to implement 2-way binding (which would be good practice anyway) on Expanded.
However, having to create a state member for each group would be a lot of effort for the consumer if they only wish to default the groups to be expanded.
Suggested solution
1: Keep Expanded
2: Rename OnExpandedChanged to ExpandedChanged in order to support 2 way binding
3: Add a new InitiallyExpanded bool parameter that is used in OnInitialized - so it defaults the state on first render, but then doesn't force a new value when the group re-renders.