Insert @defaults at start of stylesheet (#14427)

Prior to this PR, we'd put all of the `@defaults` (the CSS variables and
stuff) _after_ the `base` rules. This creates an issue when using
`optimizeUniversalDefaults` with CSS that looks like this:

```css
@tailwind base;
@tailwind components;
@tailwind utilities;

@layer base {
  input {
    @apply shadow;
  }
}
```

…because the default shadow stuff ends up after the base `input` rules,
so the generated styles are like this:

```css
input {
  --tw-shadow: 0 1px 3px 0 rgb(0 0 0 / 0.1), 0 1px 2px -1px rgb(0 0 0 / 0.1);
  --tw-shadow-colored: 0 1px 3px 0 var(--tw-shadow-color),
    0 1px 2px -1px var(--tw-shadow-color);
  box-shadow: var(--tw-ring-offset-shadow, 0 0 #0000), var(--tw-ring-shadow, 0 0 #0000),
    var(--tw-shadow);
}

input {
  --tw-ring-offset-shadow: 0 0 #0000;
  --tw-ring-shadow: 0 0 #0000;
  --tw-shadow: 0 0 #0000;
  --tw-shadow-colored: 0 0 #0000;
}
```

This means all of the actual shadow values for the input are reset and
the shadow doesn't work.

Fixes https://github.com/tailwindlabs/tailwindcss/issues/14426.

Lots of failing tests right because this changes a ton of stuff, albeit
in a totally inconsequential way. @thecrypticace if you could update
these for me this week that would be a huge help, just banging this fix
out quick while the kids are napping 😴

---------

Co-authored-by: Adam Wathan <4323180+adamwathan@users.noreply.github.com>
Co-authored-by: Jordan Pittman <jordan@cryptica.me>

tests/basic-usage.test.js

@@ -460,11 +460,11 @@ it('does not produce duplicate output when seeing variants preceding a wildcard
       * {
         color: #00f;
       }
+      ${defaults}
       .combined,
       * {
         color: red;
       }
-      ${defaults}
       .underline {
         text-decoration-line: underline;
       }