Keyboard focus improvements for font features/axes

Adding an item is generally done by clicking the "add new" button and selecting an item from the flyout. However, when you do this from the keyboard, it feels a little weird. Your focus _stays_ on the add new button.

Ideally, it would move to the newly added item.

This is particularly bad if the user added the last feature/axes. Since that disables the add new button, the button can't be focused, so it gets yeeted to the tab row.

Start by reproducing the keyboard flow for adding a font feature or axis, including adding the last available item, and inspect where focus moves after selection. Done means focus moves to the newly added item and does not fall back to the tab row when the add new button becomes disabled.

Written by the indexing model from the issue text.