[tailwindcss-vite] Add Register dependency messages logline (tailwindlabs#19611)

The PostCSS implementation has a debug logline for registering
dependencies but this is missing for @tailwindcss/vite.

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## Summary

Adds the "Register dependency messages" instrumentation span to the Vite
plugin's dependency registration code, matching the existing
implementation in @tailwindcss/postcss.

This ensures consistent debug instrumentation across both bundler
integrations when DEBUG is enabled.

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Author: timneutkens

packages/@tailwindcss-vite/src/index.ts

@@ -375,6 +375,7 @@ class Root {
     }
 
     if (this.compiler.features & Features.Utilities) {
+      DEBUG && I.start('Register dependency messages')
       // Watch individual files found via custom `@source` paths
       for (let file of this.scanner.files) {
         addWatchFile(file)
@@ -411,6 +412,7 @@ class Root {
           }
         }
       }
+      DEBUG && I.end('Register dependency messages')
     }
 
     DEBUG && I.start('Build CSS')