Ensure @tailwindcss/cli watches the input file (#20246)

This PR fixes an issue where if the `--input` file (when using
`@tailwindcss/cli`) lives in an ignored folder, then changes to the
input file won't trigger a rebuild.

This happened in #17632 where the input file lives in the `assets/`
folder which is ignored by default.

However, if you make a change to the input file, then the CLI doesn't
restart which means that you can't update the `@source` directives
without quitting the program and re-starting it manually.

This PR solves that by automatically injecting an `@source` with the
path to the input file. This way the CLI will see changes, even if the
input file is in an ignored directory.

Fixes: #17632

## Test plan

1. Added an integration test
2. Existing tests pass
3. Reproduced it on the 


Before:
<img width="1122" height="1376"
alt="file-295ce4696b6b3199a3915c63f8bb50cd"
src="https://github.com/user-attachments/assets/09e5e965-38d1-474d-bfc6-3b5e9bca73b6"
/>

After:
<img width="1122" height="1376"
alt="file-301b2f2e06aee045957355014ae5de87"
src="https://github.com/user-attachments/assets/2b6d4cba-f456-4138-98bd-2259876fe70a"
/>

packages/@tailwindcss-cli/src/commands/build/index.ts

@@ -260,6 +260,14 @@ export async function handle(args: Result<ReturnType<typeof options>>) {
       negated: true,
     })
 
+    if (inputFilePath !== null) {
+      sources.push({
+        base: path.dirname(inputFilePath),
+        pattern: path.basename(inputFilePath),
+        negated: false,
+      })
+    }
+
     let scanner = new Scanner({ sources })
     DEBUG && I.end('Setup compiler')