LibWeb: Introduce Content Security Policy policies and directives

These form the basis of Content Security Policy. A policy is a
collection of directives that are parsed from either the
Content-Security-Policy(-Report-Only) HTTP header, or the `<meta>`
element.

The directives are what restrict the operations can be performed in the
current global execution context. For example, "frame-ancestors: none"
tells us to prevent the page from being loaded in an embedded context,
such as `<iframe>`.

You can see it a bit like OpenBSD's pledge() functionality, but for the
web platform: https://man.openbsd.org/pledge.2

Libraries/LibWeb/Forward.h

@@ -98,6 +98,17 @@ class CompressionStream;
 class DecompressionStream;
 }
 
+namespace Web::ContentSecurityPolicy {
+class Policy;
+class PolicyList;
+struct SerializedPolicy;
+}
+
+namespace Web::ContentSecurityPolicy::Directives {
+class Directive;
+struct SerializedDirective;
+}
+
 namespace Web::Cookie {
 struct Cookie;
 struct ParsedCookie;