Add support for numeric focus keys in focus zones

[iansan5653]

The accessibility specification for content preview tabs (internal link) recommends supporting numeric focus keys to jump to specific items in the tabs list:

Key	Behavior
1-8	Selects the first through eighth tab in the tab list, respectively. Noop if there is not a tab in the tab list order that corresponds with the number keypress.
9	Selects the last tab in the tab list, regardless of if there are more than 9 tabs. Noop if there is only one tab.

Ideally this would be supported natively by focus-zone, I think probably as a new FocusKeys.Numeric enum value.

[TylerJDev]

Hey @iansan5653!

I agree, I think focus-zone would be a good place for this. For now I'll add this to our backlog, but let me know if there's a more immediate need for this to be added!