Only generate Preflight compatibility styles when Preflight is used (#14773)

This PR improves where we inject the border compatibility CSS. Before
this change we injected it if it was necessary in one of these spots:

- Above the first `@layer base` to group it together with existing
`@layer base` at-rules.
- If not present, after the last `@import`, to make sure that we emit
valid CSS because `@import` should be at the top (with a few
exceptions).

However, if you are working with multiple CSS files, then it could be
that we injected the border compatibility CSS multiple times if those
files met one of the above conditions.

To solve this, we now inject the border compatibility CSS with the same
rules as above, but we also have another condition:

The border compatibility CSS is only injected if the file also has a
`@import "tailwindcss";` _or_ `@import "tailwindcss/preflight";` in the
current file.

---

Added integration tests to make sure that we are generating what we
expect in a real environment. Some of the integration tests also use the
old `@tailwind` directives to make sure that the order of migrations is
correct (first migrate to `@import` syntax, then inject the border
compatibility CSS).

---------

Co-authored-by: Adam Wathan <adam.wathan@gmail.com>

Author: RobinMalfait

CHANGELOG.md

@@ -18,6 +18,7 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 - Ensure third-party plugins with `exports` in their `package.json` are resolved correctly ([#14775](https://github.com/tailwindlabs/tailwindcss/pull/14775))
 - Ensure underscores in the `url()` function are never unescaped ([#14776](https://github.com/tailwindlabs/tailwindcss/pull/14776))
 - _Upgrade (experimental)_: Ensure `@import` statements for relative CSS files are actually migrated to use relative path syntax ([#14769](https://github.com/tailwindlabs/tailwindcss/pull/14769))
+- _Upgrade (experimental)_: Only generate Preflight compatibility styles when Preflight is used ([#14773](https://github.com/tailwindlabs/tailwindcss/pull/14773))
 - _Upgrade (experimental)_: Don't escape underscores when printing theme values migrated to CSS variables in arbitrary values (e.g. `m-[var(--spacing-1_5)]` instead of `m-[var(--spacing-1\_5)]`) ([#14778](https://github.com/tailwindlabs/tailwindcss/pull/14778))
 
 ## [4.0.0-alpha.29] - 2024-10-23