Implement a DateTimePicker widget

[fnawratil]

This PR adds a new widget supporting the native datetime-local input field in browsers.

[disberd]

Nice, I think apart from the review comment about adding mention to the fallback, it would be good to add this in a easy example to the test/notebooks/basic_widgets.jl notebook, and also use that to record a small example of this in action to be added at the end of the docstring.

Since last version, we have a new orphan branch that is just used to store all assets (images, videos, html) so that it's easier to link to stuff in the docstring and documentation pages. I used to rely on pasting videos/images on PR comments and just link to that, but now all of those assets are blocked behind a JWT token which expires so can't be reliably used anymore to easily host.

The new orphaned branch is https://github.com/disberd/PlutoExtras.jl/tree/assets