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
This is required to store Content Security Policies, as their
Directives are implemented as subclasses with overridden virtual
functions. Thus, they cannot be stored as generic Directive classes, as
it'll lose the ability to call overridden functions when they are
copied.
