[SG-312492][Accessibility]: NavDropdown

Created by: gitstart-sourcegraph

Description

• Modify code to split "Code search" into a link, and the down arrow into a "MenuButton".

Refs

• SourceGraph issue
• GitStart ticket

Preview App

• Storybook
• Frontend

Test plan

• On global nav bar, the Code search menu item now contains the link and an arrow button
• Hovering on that menu item (included hovering in array only) would open sub menu
• Both the link and arrow icon can be focused via keyboard and can use keyboard buttons to open/close menu