Add the @property at-rule

[jhaygood86]

Feature

The @property at-rule (CSS Properties and Values API 1 §3) registers a typed custom property:

@property --my-color {
  syntax: '<color>';
  initial-value: red;
  inherits: false;
}

Implementation

• PropertyRule — a DeclarationRule like FontFaceRule, with its Name captured from the prelude like KeyframesRule, and Syntax / InitialValue / Inherits accessors. Exposed as IPropertyRule.
• RuleNames.Property, RuleType.Property, and the three descriptor property names.
• PropertyFactory.CreatePropertyDescriptor — the descriptors have no fixed grammar (initial-value depends on syntax; syntax is an arbitrary string), so each is stored raw via UnknownProperty.
• StylesheetComposer.CreateProperty and its dispatch in CreateAtRule.

The rule round-trips: @property --gap { syntax: "<length>"; initial-value: 0px; inherits: true }.

Scope: object model only, no registration/substitution

This adds the object model for the @property rule — parsing the prelude and the three descriptors into an inspectable, round-trippable IPropertyRule. It does not register the custom property or apply its syntax/initial-value/inherits semantics: nothing validates a var() substitution against the declared syntax, supplies the initial-value when a reference is invalid, or drives inheritance from inherits. Doing that is a computed-value/registered-property layer that would have to be built on top — the same substitution layer described in #211.

To be useful at all, this needs #211 (custom properties and var()): @property registers a custom property, so without custom-property/var() support there is nothing for it to describe. #211 is the foundation; this is the @property rule on top of it, and a substitution/registration layer would sit above both. (This branch already carries the one -- lexer change from #211 so it builds standalone — see Dependency below.)

If a registration/computed-value layer would be useful upstream, I'm happy to contribute one as a follow-up: this was extracted from a downstream renderer that already applies the @property descriptors (syntax-checked substitution, initial-value fallback, inheritance) on top of exactly these types, so a version that builds on this framework already exists and could be adapted.

Dependency

The --name prelude requires -- to lex as a single ident, which is the lexer fix from "Add custom properties and var()" (#211). This branch includes that one lexer change so it's self-contained; if #211 merges first, it rebases cleanly.

Tests

5 tests in AtPropertyTests.cs: name/descriptor capture, round-trip, the universal * syntax, IPropertyRule exposure, and an @property among style rules.

The full suite (1263 existing tests) stays green, and all seven target frameworks build with no new warnings.