Improve missing layer codemod

[RobinMalfait]

This PR improves the missing layers codemod where everything after the last Tailwind directive can stay as-is without wrapping it in a @layer directive.

The @layer at-rules are only important for CSS that exists between Tailwind directives.

E.g.:

@tailwind base;

html {}

@tailwind components;

.btn {}

@tailwind utilities;

.foo {}

.bar {}

Was transformed into:

@import "tailwindcss";

@layer base {
  html {}
}

@layer components {
  .btn {}
}

@layer utilities {
  .foo {}
  
  .bar {}
}

But the last @layer utilities is already in the correct spot, so we can simplify this to just this instead:

@import "tailwindcss";

@layer base {
  html {}
}

@layer components {
  .btn {}
}

.foo {}

.bar {}