Move button keypress and click handlers from the slot to the button part. With this change, the split-button no longer needs to preventDefault to make only the arrow section of the button open the listbox.
Change split-button listbox style so that the current option is highlighted instead of watching for checked. This way there's a visual indicator when using up/down arrows to select a value.
Move listbox controller code from the listbox-container slot to the listbox part. Refactor so that the same functions are used to apply controller code to both the default parts and the user-supplied parts, to ensure that these remain consistent.
Make the text half of the button focusable, and update the focus ring style to match behavior at https://getbootstrap.com/docs/4.0/components/dropdowns/#split-button-dropdowns
