add rewrite rule for css-extensions

Author: Peter Linss

.htaccess

@@ -129,6 +129,7 @@ RewriteRule ^css-counter-styles-3/(.*)$ css-counter-styles/$1
 RewriteRule ^css-device-adapt-1/(.*)$ css-device-adapt/$1
 RewriteRule ^css-display-3/(.*)$ css-display/$1
 RewriteRule ^css-exclusions-1/(.*)$ css-exclusions/$1
+RewriteRule ^css-extensions-1/(.*)$ css-extensions/$1
 RewriteRule ^css-flexbox-1/(.*)$ css-flexbox/$1
 RewriteRule ^css-font-loading-3/(.*)$ css-font-loading/$1
 RewriteRule ^css-fonts-3/(.*)$ css-fonts/$1