Add custom properties and var()

[jhaygood86]

Feature

Custom properties and var() references (CSS Custom Properties for Cascading Variables 1):

:root { --main-color: red; --gap: 10px; }
.a { color: var(--main-color); margin: var(--gap) 5px; }

On master: --foo lexes as a - delimiter plus -foo, custom-property declarations are dropped, and any value containing var() is rejected against the property's own grammar.

Changes

• Lexer — a second - also begins an ident, so --foo is one ident (fixed in both the top-level - dispatch and IdentStart). --> still closes an HTML-style comment.
• CustomProperty — created for any --* name via PropertyFactory.IsCustomPropertyName, with its value kept verbatim (Converters.Any). - and -- alone are reserved, not custom properties.
• var() on any property — Property.TrySetValue routes a value containing var() through Converters.Any rather than the property grammar, since the referenced value is only known per-element at cascade time (§3).
• var() in a shorthand — kept whole rather than sliced into longhands (both StyleDeclaration.SetShorthand and the StylesheetComposer declaration path), because it can only be split after substitution (§3.2). margin: var(--gap) 5px stays a single margin declaration.
• ValueExtensions.ContainsFunction — detects a function call anywhere, including nested (var() inside calc()).

Scope: object model only, no substitution

This adds the object model for custom properties and var() — parsing, storage, and serialization — so a stylesheet round-trips faithfully and the declarations are queryable. It does not include a var() substitution layer: a property that references var() keeps its value verbatim (e.g. color reports var(--c)), it is not resolved to the referenced custom property's computed value.

Fully resolving a stylesheet — walking the cascade per element, substituting each var() reference (honouring fallbacks and the guaranteed-invalid / cyclic-reference rules of CSS Variables 1 §3), then re-expanding any shorthand that contained one — is a separate cascade/computed-value layer that would have to be built on top of this. That layer is out of scope here; this PR is only the parser/object-model foundation it would consume.

If a substitution layer would be useful upstream, I'm happy to contribute one as a follow-up: this object model was extracted from a downstream renderer that already implements var() substitution on top of exactly these types, so a version that builds on this framework already exists and could be adapted.

Tests

19 cases in PropertyTests/CustomPropertyTests.cs: custom properties stored verbatim (including odd values), the CustomProperty type, IsCustomPropertyName (rejecting -/--), var() accepted on longhands and inside calc(), var()-bearing shorthands kept whole with longhands unpopulated, and the --foo-is-one-ident lexer regression.

The full suite (1263 existing tests) stays green — the -- lexer change and var() routing disturb nothing — and all seven target frameworks build with no new warnings.

[jhaygood86]

Rebased onto current master.

The --foo-is-one-ident lexer change this branch carried as a prerequisite is already on master (it went in with #212), so it has dropped out of the diff entirely. What's left is just the custom-property object model: CustomProperty, PropertyFactory.IsCustomPropertyName/CreateCustomProperty, ValueExtensions.ContainsFunction, the var() routing in Property.TrySetValue, and keeping a var()-bearing shorthand whole in StyleDeclaration/StylesheetComposer.

The "Dependency" section of the description above is therefore obsolete — there's nothing left to rebase around. The scope note still stands: this is the object model only, no var() substitution layer.