Add @source support (tailwindlabs#14078)

This PR is an umbrella PR where we will add support for the new
`@source` directive. This will allow you to add explicit content glob
patterns if you want to look for Tailwind classes in other files that
are not automatically detected yet.

Right now this is an addition to the existing auto content detection
that is automatically enabled in the `@tailwindcss/postcss` and
`@tailwindcss/cli` packages. The `@tailwindcss/vite` package doesn't use
the auto content detection, but uses the module graph instead.

From an API perspective there is not a lot going on. There are only a
few things that you have to know when using the `@source` directive, and
you probably already know the rules:

1. You can use multiple `@source` directives if you want.
2. The `@source` accepts a glob pattern so that you can match multiple
files at once
3. The pattern is relative to the current file you are in
4. The pattern includes all files it is matching, even git ignored files
1. The motivation for this is so that you can explicitly point to a
`node_modules` folder if you want to look at `node_modules` for whatever
reason.
6. Right now we don't support negative globs (starting with a `!`) yet,
that will be available in the near future.

Usage example:

```css
/* ./src/input.css */
@import "tailwindcss";
@source "../laravel/resources/views/**/*.blade.php";
@source "../../packages/monorepo-package/**/*.js";
```

It looks like the PR introduced a lot of changes, but this is a side
effect of all the other plumbing work we had to do to make this work.
For example:

1. We added dedicated integration tests that run on Linux and Windows in
CI (just to make sure that all the `path` logic is correct)
2. We Have to make sure that the glob patterns are always correct even
if you are using `@import` in your CSS and use `@source` in an imported
file. This is because we receive the flattened CSS contents where all
`@import`s are inlined.
3. We have to make sure that we also listen for changes in the files
that match any of these patterns and trigger a rebuild.

PRs:

- [x] tailwindlabs#14063
- [x] tailwindlabs#14085
- [x] tailwindlabs#14079
- [x] tailwindlabs#14067
- [x] tailwindlabs#14076
- [x] tailwindlabs#14080
- [x] tailwindlabs#14127
- [x] tailwindlabs#14135

Once all the PRs are merged, then this umbrella PR can be merged. 

> [!IMPORTANT]  
> Make sure to merge this without rebasing such that each individual PR
ends up on the main branch.

---------

Co-authored-by: Philipp Spiess <hello@philippspiess.com>
Co-authored-by: Jordan Pittman <jordan@cryptica.me>
Co-authored-by: Adam Wathan <adam.wathan@gmail.com>

Author: 4 people

CHANGELOG.md

@@ -11,6 +11,7 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 - Add support for `inline` option when defining `@theme` values ([#14095](https://github.com/tailwindlabs/tailwindcss/pull/14095))
 - Add `inert` variant ([#14129](https://github.com/tailwindlabs/tailwindcss/pull/14129))
+- Add support for explicitly registering content paths using new `@source` at-rule ([#14078](https://github.com/tailwindlabs/tailwindcss/pull/14078))
 
 ## [4.0.0-alpha.18] - 2024-07-25