Refactored ContextMenu into it's own control to allow displaying in
CommandBar and in response to right click on list items.
- Adds "critical" styling to context menu items flagged as `IsCritical`.
This will use the theme to style with correct color.
- Added `SeparatorContextItem` and modified `MoreCommands` to allow for
both `CommandContextItem`s and `SeparatorContextItem`s.
- Right clicking a list item with a context menu will open the context
menu at the position of the click and position the filter box at the top
of the context menu.
This PR covers:
- closes#38308
- closes#39211
- closes#38307
- closes#38261
