Add support for automatic JSX runtime

[dcastil]

Closes #1160

React introduced the new JSX transform and Emotion added support in emotion-js/emotion#1970. So everything is in place for Theme UI to support the new JSX transform. 😊

To dos:

• Resolve discussions I added below.
• Test automatic JSX runtime manually
• Write tests
• Update documentation

[shadcn]

This commit thread is a roller coaster of emotion (pun intended?)

fml tpyescript is so hard
why am I doing this to myself

Thanks for putting in the work for the rest of us @dcastil 👍

[dcastil]

@arshad Haha yes, I was fighting TypeScript and as I fixed type bugs, new came up again and again. 😄 But now it works!

[hasparus]

I'll handle the conflicts and changelog.

[dbertella]

@dcastil is there any additional library to install in order to use jsxImportSource because at the moment I can't get it to work with the example in the readme. I opened an issue here if you need more context #1474 not sure if I'm just missing something very obvious here. Thanks!

[hasparus]

@dbertella's comment is already answered in another issue, but I'll leave this here for posterity.

To use Theme UI with automatic runtime you need Theme UI version greater than 0.5.0-alpha.

However, classic runtime still works, so if you're already using it you don't need to migrate, it's possible to configure this in your Babel config or by setting DISABLE_NEW_JSX_TRANSFORM=true env var in CRA (see #1474 (comment)).