Add `selectionGroup` to TreeItem to allow for multiselect restrictions

Currently allowing multiselect on a TreeView is an all or nothing affair, and if the view contains items of different types that leads to a less than desirable user experience. I'm starting to enable multiselect in GitLens and here is one example where I've enabled multiselect on the Stashes view  The selection now contains files & stashes, which grouped together as shown don't really make sense. And currently extensions can't limit selection in any way. I'm proposing the addition of a new optional string `selectionGroup` (or even `selectionContext` property on `TreeItem`s to allow extensions to group/bound the items that can be selected. So in the example above, I could have the "file" items have a `selectionGroup` of `"files"` and the "stashes" a `selectionGroup` of `"stashes"`. And then based on the first selected item, you would only be able to select other items in that same group. So if I started on a "file" I could only select other files, etc. We may also want to allow the `selectionGroup` to be set to `*` if there were items you wanted to allow in any group. This could be taken further, to have 2 props, one for the `selectionGroup` of the item itself, but another `allowedSelectionGroups` which could be a string array of all the selection groups that an item could belong to. I think starting with `selectionGroup` alone gets us pretty far and imo would start there. //cc @alexr00