Add CSS Images 4 image functions and fix value-context hash tokenization

[jhaygood86]

Feature

image-set(), cross-fade() and element() (CSS Images 4 §2/§3) are valid <image> values but were rejected:

background-image: image-set("a.png" 1x, "b.png" 2x);
background-image: cross-fade(url(a.png), url(b.png), 50%);
background-image: element(#hero);

Adds ImageSetConverter, CrossFadeConverter and ElementImageConverter, composed into ImageSourceConverter so every <image> property accepts them.

Bundled lexer fix (required by element(#id))

element(#hero) needs a working <hash-token> in a value context, which surfaced a lexer bug. In a value context # was treated as a colour literal that consumed only hex characters, so #hero tokenized as an empty Color plus a stray hero Ident.

CSS Syntax §4.3.4 says # begins a whole <hash-token> consuming an entire <name>. The fix classifies it as a colour only when the name is all-hex (#f00, #abc123), otherwise emits a Hash token (#hero, #f00bar); a # not followed by a name is a plain delimiter. ValueBuilder now accepts Hash tokens in a value.

x resolution unit

image-set() options idiomatically use 1x/2x. x is the canonical alias for dppx (CSS Values 4 §7.4); it was missing from Resolution.GetUnit.

Tests

• Tokenization.cs: #f00/#abc123/#deadbeef → Color; #hero/#top/#f00bar → Hash; # alone → delimiter
• PropertyTests/BackgroundProperty.cs: 5 legal (image-set string & url with 1x/2dppx, both cross-fade forms, element(#hero)) and 3 malformed (image-set(banana), element(.klass), cross-fade(5px))

The full suite (1263 existing tests) stays green — the value-context hash change doesn't disturb any colour parsing — and all seven target frameworks build with no new warnings.