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"
/>

integrations/cli/index.test.ts

@@ -404,6 +404,57 @@ describe.each([
     },
   )
 
+  // https://github.com/tailwindlabs/tailwindcss/issues/17632
+  test(
+    'watch mode rebuilds when the input file is in an ignored folder',
+    {
+      fs: {
+        'package.json': json`
+          {
+            "dependencies": {
+              "tailwindcss": "workspace:^",
+              "@tailwindcss/cli": "workspace:^"
+            }
+          }
+        `,
+        'assets/tailwind.config.js': js`
+          module.exports = {
+            content: {
+              relative: true,
+              files: ['html/*.html'],
+            },
+          }
+        `,
+        'assets/html/index.html': html`
+          <div class="underline"></div>
+        `,
+        'assets/css/index.css': css`
+          @import 'tailwindcss' source(none);
+          @config '../tailwind.config.js';
+        `,
+      },
+    },
+    async ({ fs, spawn }) => {
+      let process = await spawn(
+        `${command} --input assets/css/index.css --output dist/out.css --watch`,
+      )
+      await process.onStderr((m) => m.includes('Done in'))
+
+      await fs.expectFileToContain('dist/out.css', [candidate`underline`])
+
+      await fs.write(
+        'assets/css/index.css',
+        css`
+          @import 'tailwindcss' source(none);
+          @config '../tailwind.config.js';
+          @source inline("flex");
+        `,
+      )
+
+      await fs.expectFileToContain('dist/out.css', [candidate`flex`])
+    },
+  )
+
   test(
     'production build (stdin)',
     {