Feat: `Date` + `Time` Picker

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`](https://github.com/alanchan-dev/OmniDateTimePicker/blob/b9712db38cb1aa91a633106f95050f79db1c52b2/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: https://github.com/dwyl/flutter-date-time-tutorial/issues/1
+ [ ] Once this repo is populated, we can pull this code into the `App`.