Improve color scheme selector

[uudens]

Replace some custom elements in the Example files with React Spectrum elements in order to get color scheme selector working properly

[mburri]

Looks very nice! but there is an issue with the example picker that is out of sync with the shown example, if the theme changes.

How to reproduce:

• select an example, e.g. "numbers"
• switch theme

Behavior: the "numbers" example is still selected, but a different example is shown.

[uudens]

@mburri thank you for the feedback and for checking it out! My bad, I've pushed a commit with a fix 🙂

[mburri]

On a closer look, I noticed that new code was not formatted with prettier and the license header was missing in the new file theme.switcher.tsx - I allowed myself to fix it.

I think the prettier formatting is not documented and it would be a good idea to add a commit hook that asserts the prettier formatting - see #71

@uudens feel free to merge this PR.

[uudens]

@mburri thanks!