fix: skip processing of remote stylesheets (#75)

Previously, we tried to read remote stylesheets using `readFile`.

Related to angular/angular-cli#20794

Co-authored-by: Jason Miller <developit@users.noreply.github.com>

Author: alan-agius4

packages/critters/src/index.js

@@ -240,6 +240,12 @@ export default class Critters {
         .substring(pathPrefix.length)
         .replace(/^\//, '');
     }
+
+    // Ignore remote stylesheets
+    if (/^https:\/\//.test(normalizedPath) || href.startsWith('//')) {
+      return undefined;
+    }
+
     const filename = path.resolve(outputPath, normalizedPath);
 
     let sheet;