CSS templates

css name(params) { … } declares a scoped class; property values may be constant or { expr } (sanitized). Use via class={ name(...) }.

css warning(color string) {
	background-color: #ffeeee;
	color: { color };
}

AST: CSSTemplate, CSSProperty, ConstantCSSProperty, ExpressionCSSProperty.

Verified by: internal/generator/test-css-expression, internal/generator/test-css-usage, internal/generator/test-css-middleware, conformance.