Fix menu item indentation bug with dynamically slotted icons

[m-akinc]

Pull Request

🤨 Rationale

Fixes #2266

👩‍💻 Implementation

Proper indentation relies on startColumnCount being set on the menu items. This is handled by FoundationMenu.setItems. It needs to be called whenever the children of the slotted menu items change. To do this, we need to modify the template so that it watches for any added/removed icons that are children of its menu items (via children directive). I've forked the FAST Menu class (into separate file) and the template to modify them.

These modifications are in menu.foundation.ts rather than index.ts because this is the kind of bug fix that we would have upstreamed to FAST.

🧪 Testing

Added unit tests and forked FAST Menu tests.

✅ Checklist

• I have updated the project documentation to reflect my changes or determined no changes are needed.

[m-akinc]

@mollykreis will you please buddy this change?