Simplify @variant usage, allow compound and stacked variants (#19996)

This PR improves and simplifies the `@variant` usage. 

When we originally added support for `@variant`, we wanted to keep
things simple, where we could only use a single variant at a time. The
original PR did have a more complex system with all these features
enabled, but we wanted to make sure that we only introduced the
additional complexity when the community felt like it was needed.

But of course we still wanted to make sure that you could do compound
and stacked variants, it just required some additional code.

For compound variants, where you want to use variant `a` and variant
`b`, you could duplicate the rules as siblings:
```css
.foo {
  @variant a {
    display: flex;
  }

  @variant b {
    display: flex;
  }
}
```

But with this PR, you can comma separate each variant to get the same
effect:
```css
.foo {
  @variant a, b {
    display: flex;
  }
}
```
You can think of this as-if we are expanding this syntax into the
aforementioned syntax. In other words, we would do the duplication for
you.

Additionally, you also want to be able to stack variants. For that you
had to nest your `@variant` rules:
```css
.foo {
  @variant a {
    @variant b {
      display: flex;
    }
  }
}
```

Not the end of the world, but it can get pretty nested if you want to
use multiple variants. Luckily we already have a syntax for this in
normal Tailwind CSS classes: `a:b:flex`. Which is exactly what we can
use here as well:
```css
.foo {
  @variant a:b {
    display: flex;
  }
}
```
Again, conceptually you can think of this syntax being expanded into the
syntax from above.

Last but not least, we can also combine these:
```css
.foo {
  background: black;

  @variant a, b:c {
    background: red;

    @variant d, e:f {
      background: blue;
    }
  }
}
```
This conceptually translates into the much more verbose version today:
```css
.foo {
  background: black;

  @variant a {
    background: red;

    @variant d {
      background: blue;
    }

    @variant e {
      @variant f {
        background: blue;
      }
    }
  }

  @variant b {
    @variant c {
      background: red;

      @variant d {
        background: blue;
      }

      @variant e {
        @variant f {
          background: blue;
        }
      }
    }
  }
}
```

The biggest downside is that this could potentially easily balloon your
CSS file size if you're not careful. Because with this, it's pretty easy
to add one more variant that introduces a lot of duplicated CSS.

This feature is completely backwards compatible, you can still nest your
`@variant` calls yourself if you want, and combine them with these
features if you want.

This is also a continuation of #19526 and #19884, but for some reason I
don't have push rights, so I'm creating this new PR instead. I did keep
the original commits of those PRs so these contributors are still
properly marked as contributors.
<img width="808" height="135" alt="image"
src="https://github.com/user-attachments/assets/bee334ab-39d7-4d4d-a48f-afa2253cf17b"
/>
<img width="349" height="75" alt="image"
src="https://github.com/user-attachments/assets/fb68906c-db74-43f7-83e4-918ad3d4a036"
/>


Closes: #19526
Closes: #19884

## Test plan

1. Added a bunch of new tests to verify this new behavior
2. Added tests that compare the short (new) version, and the long (old)
version
3. Added a sourcemap related test to ensure that the src and dst
locations are correct
4. Existing tests still pass

---------

Co-authored-by: orteth01 <tortega128@gmail.com>
Co-authored-by: Ray Knight <array.knight+github@gmail.com>

Author: 3 people

CHANGELOG.md

@@ -10,6 +10,8 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 ### Added
 
 - _Experimental_: add `@container-size` utility ([#18901](https://github.com/tailwindlabs/tailwindcss/pull/18901))
+- Allow using `@variant` with stacked variants (e.g. `@variant hover:focus { … }`) ([#19996](https://github.com/tailwindlabs/tailwindcss/pull/19996))
+- Allow using `@variant` with compound variants (e.g. `@variant hover, focus { … }`) ([#19996](https://github.com/tailwindlabs/tailwindcss/pull/19996))
 
 ### Fixed