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

Start by locating the modal integration and reviewing the existing Escape-key behavior described in this issue. Check the linked vex upstream pull request and issue discussion before choosing a workaround. Done means the modal’s close control can receive keyboard focus while retaining its current close behavior.

Written by the indexing model from the issue text.