feat(react-email): added a theme switcher to the dev preview

[KayleeWilliams]

I added a theme switcher to the developer preview. This should allow for an easier/more convient time when designing emails to look good in both light & dark environments.

This only sets the color scheme in the CSS. The actual email will still need to add support whether using Talwind's dark: attribute or setting it in the style.

Below is an altered version of the Vercel example which has dark-mode support (except the logo)

Screen.Recording.2024-10-29.at.23.24.11.mov

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[vercel]

@KayleeWilliams is attempting to deploy a commit to the resend Team on Vercel.

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[gabrielmfern]

Thanks for this! We were already planning on having something similar implemented, so this is a good starting point.

[gabrielmfern]

Requested a few changes, not a lot, just some implementation details

[KayleeWilliams]

Requested a few changes, not a lot, just some implementation details

Made some tweaks, changed it to the iFrame as suggested but removed the Theme Provider entirely as it was just doing the same job as the search params, so wasn't necessary.