v5: Update colors to add shades and tints (#29348)

* Add variables for shades and tints of each major color

* getting-started/theming.md: use a `range`.

scss/_functions.scss

@@ -100,3 +100,11 @@
 
   @return mix($color-base, $color, $level * $theme-color-interval);
 }
+
+@function tint-color($color, $level) {
+  @return mix(white, $color, $level * $theme-color-interval);
+}
+
+@function shade-color($color, $level) {
+  @return mix(black, $color, $level * $theme-color-interval);
+}