DatePickerE: Add focus trap

[jpveooys]

From @k9dh3zij (in #2841):

Can we make it so that the Sheet contains its own tab sequence. (Tab and Shift + Tab do not move focus outside the dialog)

This is based on https://www.w3.org/TR/wai-aria-practices/examples/dialog-modal/datepicker-dialog.html

There is a library that can do this: https://github.com/focus-trap/focus-trap-react

However, from testing, there seems to be a bug in the underlying library (https://github.com/focus-trap/focus-trap) – when a date after the first of the month is focused, the tab key stops functioning (while Shift+Tab continues to work).

This needs further investigation and a bug to be reported.

[jpveooys]

I've opened an issue regarding the problem encountered with focus-trap here: focus-trap/focus-trap#565

[jpveooys]

Previous work on this is on this branch: https://github.com/defencedigital/mod-uk-design-system/tree/feature/date-picker-e-focus-trap