feat: contrast util with yiq calculator (#3652)

* add yiq calculator util

* fix: convert 3 chars hex to 6 chars hex

* fix: clarify util name

* feat: add text color variables not depending on the dark/light mode

* refactor: change getContrast to isDark with for a more direct approach

* fix: adjust snippet description

* chore: change `var` to `let`

Co-authored-by: David Wheatley <david@davwheat.dev>

framework/core/less/common/variables.less

@@ -64,6 +64,13 @@
   @code-color:                     #fff;
 }
 
+// Beyond dark or light mode, we need stable colors depending on the luminosity
+// of the parents element's background. This allow not to change the color
+// variable depending on the dark/light mode to get the same final color, and
+// thus to simplify the logic.
+@text-on-dark:                     #ddd;
+@text-on-light:                    #111;
+
 @hero-bg:                          @control-bg;
 @hero-color:                       @control-color;
 @hero-muted-color:                 @control-color;