Support @page named selectors, selector lists, size, and the page property

[jhaygood86]

Feature

CSS Paged Media 3 named page selectors, selector lists, the size descriptor and the page property:

@page chapter { margin: 1cm; }
@page chapter1:left, chapter2:left { margin: 1cm; }
@page { size: A4; }
@page { size: 8.5in 11in; }
.cover { page: chapter; }

On master, @page accepts only a single :pseudo selector — a named page or a selector list is dropped — and neither size nor page is recognised.

Implementation

• PageSelector now holds a list of PageSelectorEntry (Name + Pseudo), and CreatePageSelector parses the §4.1 grammar per comma-separated entry: an optional <ident> page name then an optional :<ident> pseudo-class. @page :left still works.
• The size @page descriptor (§6.3) — a page-size/orientation keyword (A4, letter, landscape) or one or two lengths — and the page property (§3.4), with StyleDeclaration.Size / PageName accessors.

Tests

14 cases in PropertyTests/PageRuleTests.cs: six selector forms (pseudo-only, name-only, name+pseudo, multi-entry lists), the exposed PageSelectorEntry list, five size values, and the page property.

The full suite (1263 existing tests) stays green — including the existing @page :left { size:A4; … @bottom-right{…} } test — and all seven target frameworks build with no new warnings.

PageSelector's constructor changed from PageSelector(string) to PageSelector(IEnumerable<PageSelectorEntry>). It's a public type; flagging the source-compat change, though nothing in the library or tests constructed it directly.