style: Minor cleanup of our @page rule setup.
style: Minor cleanup of our @page rule setup.

Actually, there's not so much we can improve right now, in the sense
that:

 * We need the ::-moz-page-content pseudo-element to be able to set
 `display` on the page, since that's a style rule rather than a @page
 rule. We could get away without it.

 * Keeping the current code-path (slightly cleaned up) is less code, for
 now at least. We can have a separate code-path or what not that
 actually performs the @page rule selector-matching and what not if
 needed when we get to named pages or other page selectors. Selectors
 like :first should be pretty trivial to implement, actually.

We make some paged mode anon boxes non-inheriting anon boxes. This
allows us to share the styles and is generally nicer. They don't need to
inherit from anywhere.

We could remove the origin handling and don't look at UA rules or what
not, but it seems pretty harmless to do that.

We also fix the name of the pseudo-elements to match the capitalization.

Differential Revision: https://phabricator.services.mozilla.com/D104772

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