LibWeb/CSP: Implement the script-src-elem directive

Author: https://github.com/Lubrsi
Commit: f382bccc3d
Pull-request: https://github.com/LadybirdBrowser/ladybird/pull/5404
Reviewed-by: https://github.com/shannonbooth ✅

Libraries/LibWeb/Forward.h

@@ -141,6 +141,7 @@ class ManifestSourceDirective;
 class MediaSourceDirective;
 class ObjectSourceDirective;
 class ScriptSourceDirective;
+class ScriptSourceElementDirective;
 struct SerializedDirective;
 
 }