[PR] Creating basic demo on both inline and Material datetime pickers

[LuchoTurtle]

closes #1

This PR implements both Material (through datePicker and timePicker) and inline scenarios for mutating DateTime objects in Flutter.
The README documents the process on both occasions and is also tested.

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[LuchoTurtle]

@nelsonic
This should be reviewable and mergeable.
I've added tests to cover each scenario and GH workflow to run these tests.
I've also added a simple incrementing counter to showcase how easy it is to use callbacks to track how many times the person is updating the dateTime field.

I didn't add a deploy workflow. I want to leave this for a separate PR.

[nelsonic]

@LuchoTurtle Looks good. Thanks.👌

[panoramix360]

👀

That's really nice guys!

[LuchoTurtle]

It's just a simple demo @panoramix360 but thanks 😍