fix(build): do not warn when URL in CSS is externalized

[fi3ework]

Description

fix #10766.

Do not warn when a URL match externalized in build.rollupOptions.external.

Additional context

---

What is the purpose of this pull request?

• Bug fix
• New Feature
• Documentation update
• Other

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[xavi7th]

How can one use this option to define external urls?

[xavi7th]

For anyone else who wonders how to implement this, here is how I went about this

  return defineConfig( {
    build: {
      rollupOptions: {
        external: [
          /^\/build\/vendor/, /^\/assets\/inertiajs/, /^\/sales-funnel/, /^\/conferences/, /^\/img\//, /^\/beyond-four-walls-conference/, /^\/accelerator-programme/,
          /^\/images\/slider\//, /^\/fonts\/slick/, /^\/img\//
        ]
      }
    },
   // ... other options here
  }

The external option can take either a string, a Regex or an array of strings (the full path you are trying to ignore) and/or Regexes (like I used above)