Rich text editor | Nimble listbox component bring up for mention popup

[vivinkrishna-ni]

Pull Request

🤨 Rationale

The nimble-rich-text-mention-list-box is the component used to populate the list of mention options in the editor. To enable some of the interactions within the popup, it requires FAST's listbox component. As the listbox has not been created in Nimble yet, this PR introduces a new nimble-listbox component.

For more details about mention list box component, see mention-hld.

👩‍💻 Implementation

1. Create a new nimble-listbox component from fast-listbox
2. Add to the Tests/ section in the storybook as it is not intended for end users usage and it is for the implementation of nimble-rich-text-mention-list-box for mention popup.
3. Create menuMinWidth token to share the min-width between menu and listbox

🧪 Testing

1. Basic test cases are written for the new nimble-listbox component
2. Storybook matrix test

✅ Checklist

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