A11y: Modal close "button" should be keyboard focusable

We can't focus the "x" close button via keyboard:

We can still close the modal via escape key: https://github.com/bbcmicrobit/PythonEditor/pull/310

We could fix this via workaround, or if the modal library get's a fix upstream: https://github.com/HubSpot/vex/pull/134

More info in https://github.com/bbcmicrobit/PythonEditor/issues/347#issuecomment-728986198