Add initial codemod tooling

[RobinMalfait]

This PR adds some initial tooling for codemods. We are currently only interested in migrating CSS files, so we will be using PostCSS under the hood to do this. This PR also implements the "migrate @apply" codemod from #14412.

The usage will look like this:

npx @tailwindcss/upgrade

You can pass in CSS files to transform as arguments:

npx @tailwindcss/upgrade src/**/*.css

But, if none are provided, it will search for CSS files in the current directory and its subdirectories.

≈ tailwindcss v4.0.0-alpha.24

│ No files provided. Searching for CSS files in the current
│ directory and its subdirectories…

│ Migration complete. Verify the changes and commit them to
│ your repository.

The tooling also requires the Git repository to be in a clean state. This is a common convention to ensure that everything is undo-able. If we detect that the git repository is dirty, we will abort the migration.

≈ tailwindcss v4.0.0-alpha.24

│ Git directory is not clean. Please stash or commit your
│ changes before migrating.

│ You may use the `--force` flag to override this safety
│ check.

---

This PR alsoo adds CSS codemods for migrating existing @apply directives to the new version.

This PR has the ability to migrate the following cases:

---

In v4, the convention is to put the important modifier ! at the end of the utility class instead of right before it. This makes it easier to reason about, especially when you are variants.

Input:

.foo {
  @apply !flex flex-col! hover:!items-start items-center;
}

Output:

.foo {
  @apply flex! flex-col! hover:items-start! items-center;
}

---

In v4 we don't support !important as a marker at the end of @apply directives. Instead, you can append the ! to each utility class to make it !important.

Input:

.foo {
  @apply flex flex-col !important;
}

Output:

.foo {
  @apply flex! flex-col!;
}