Accessibility: Keyboard focus styling isn't very visible

[Raymond Zhao]

1. Place the component on a webpage and tab to it.
2. 🐛 There is some styling on focus, but it's not that obvious, especially on the left half.

A way to make keyboard focus more obvious is to add an outline border.

Example screencap:

[fsteffi]

primary bug: microsoft/debug-adapter-protocol#206