Register the ::marker pseudo-element

[jhaygood86]

Problem

li::marker { color: red } fails to parse — sheet.Rules.Length is 0, the whole rule is dropped.

::marker is a standard pseudo-element (CSS Lists 3 §6.1), but it is missing from the registered set in PseudoElementSelectorFactory:

new HashSet<string>(StringComparer.OrdinalIgnoreCase)
{
    PseudoElementNames.Before,
    PseudoElementNames.After,
    // PseudoElementNames.Marker  <-- absent
    PseudoElementNames.Selection,
    PseudoElementNames.FirstLine,
    PseudoElementNames.FirstLetter,
    PseudoElementNames.Content,
}

So Create("marker") returns null (unless AllowInvalidSelectors is set), and the selector — and its rule — is discarded.

Fix

Add PseudoElementNames.Marker = "marker" and register it in the factory, exactly like the other pseudo-elements. It now parses to a PseudoElementSelector with Name == "marker" and round-trips as ::marker.

Only the two-colon form is registered. Unlike ::before/::after, ::marker has no one-colon legacy spelling (it postdates CSS2), so :marker stays invalid.

Scope note

This is the parse fix only. In the fork this came from, ::marker registration rode along with a larger rework that also gives :first-child/:only-child/etc. dedicated selector subtypes — but those already parse correctly here (identical text and specificity), so that part is an object-model change specific to that fork's matching engine, not a parsing or spec-conformance fix, and is left out.

Tests

3 tests in SelectorsTests.cs: li::marker and bare ::marker parse and round-trip, and the subject is a PseudoElementSelector named marker. All 3 fail on master. The full suite (1263 existing tests) stays green, and all seven target frameworks build with no new warnings.