feat: add --aura-neutral light-dark color (#10642) (#10647)

Co-authored-by: Jouni Koivuviita <jouni@vaadin.com>

Author: vaadin-bot

packages/aura/src/color.css

@@ -82,6 +82,7 @@ vaadin-tab,
   --aura-neutral-dark: oklch(
     from var(--aura-background-color-dark) calc(0.9 + 0.1 * var(--aura-contrast-level)) calc(c * l) h
   );
+  --aura-neutral: light-dark(var(--aura-neutral-light), var(--aura-neutral-dark));
   --vaadin-text-color: light-dark(var(--aura-neutral-light), var(--aura-neutral-dark));
   --vaadin-text-color-secondary: color-mix(
     in oklch,