Feat: `Date` + `Time` Picker

[nelsonic]

At present in the MVP datetime for a timer can be edited as plaintext:

This is fine for MVP as it allows us to test that people are editing timer.start and/or timer.stop.

The following video from HeyFlutter summarises the Native approach to date and time picking in 5 mins:
"Pick Date And Time Together": https://youtu.be/Y_1SHNI__6I

https://github.com/alanchan-dev/OmniDateTimePicker looks like a clean interface:

But sadly, doesn't have tests at present: /test/omni_datetime_picker_test.dart#L1-L8
So we can use it as a reference but can't just use the code - unless we write our own tests ...

Todo

• Quickly create a sample project to showcase date and time input: https://github.com/dwyl/flutter-date-time-tutorial
• Document all the steps required to use existing native widgets (that work on Mobile, Web & Desktop) to input date and time data. Ideally in a single interface similar to the above: Feat: Setup + Document date and time input in a Flutter App flutter-date-time-tutorial#1
• Once this repo is populated, we can pull this code into the App.