Forms: allow switching between themed and browser native date pickers

[simison]

Browser's date picker is now widely supported and pretty good and has excellent accessibility out of the box.

It only lacks customization options for styles, but it already has some.

We should consider defaulting to it soon, but meanwhile, we can add it as a Style option to the Date block.

Themed

Browser native

There could be better style names than these, but just to illustrate the idea:

I considered "Styled" and "Unstyled" but it's not quite correct since you can apply some styles to the native one, too.