fix(tailwind): User-defined CSS variables being replaced with undefined

[gabrielmfern]

This is meant to address #1448.

The problem there was that, in a nutshell, CSS variables defined in the
template, and then used through Tailwind utilities, would end up being replaced
with just undefined.

This happens due to the behavior of
postcss-css-variables,
the PostCSS plugin we use internally ourselves to inline Tailwind's CSS
variables. The issue then happens when this library finds a CSS variables that
it can't find a declaration for, making it just undefined. This behavior is
questionable together with a few other problems that cause bugs that we need
patches for currently. The library though, does not have any configuration to
allow for us to override this behavior, and patching it would be just too
cumbersome to both maintain and to make in the first place.

With that in mind, this PR removes the use of that dependency and writes in a
new, relatively naive, implementation of a PostCSS plugin that we can maintain
and adjust the behavior of ourselves. It also adds in a few unit tests to
ensure the behavior of this new plugin is equivalent to the old one we used.

[vercel]

The latest updates on your projects. Learn more about Vercel for Git ↗︎

Name	Status	Preview	Comments	Updated (UTC)
react-email	✅ Ready (Inspect)	Visit Preview	💬 Add feedback	Sep 16, 2024 4:01pm
react-email-demo	✅ Ready (Inspect)	Visit Preview	💬 Add feedback	Sep 16, 2024 4:01pm

[changeset-bot]

🦋 Changeset detected

Latest commit: 2952971

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 13 packages

Name	Type
@react-email/tailwind	Patch
@react-email/components	Patch
react-email-with-aws-ses	Patch
react-email-with-mailersend	Patch
react-email-with-nodemailer	Patch
react-email-with-plunk	Patch
react-email-with-postmark	Patch
react-email-with-resend	Patch
react-email-with-sendgrid	Patch
react-email-with-next-scaleway	Patch
react-email-with-node-scaleway	Patch
react-email-starter	Patch
create-email	Patch

Not sure what this means? Click here to learn what changesets are.

Click here if you're a maintainer who wants to add another changeset to this PR

[papidb]

Hi, has this been pushed?