fix(material-experimental/theming): incorrect validation for density scale

We were missing a comma in the list of allowed densities which was breaking the validation.

Author: crisbeto

src/dev-app/theme-m3.scss

@@ -149,7 +149,7 @@ html {
 }
 
 // Emit density styles for each scale.
-@each $scale in (maximum, 0, -1, -2, -3, minimum) {
+@each $scale in (maximum, 0, -1, -2, -3, -4, minimum) {
   $scale-theme: matx.define-theme(map.set($m3-base-config, density, scale, $scale));
 
   .demo-density-#{$scale} {

src/material-experimental/theming/_config-validation.scss

@@ -154,7 +154,7 @@
     @return (#{'$config has unexpected properties. Valid properties are: scale. Found:'} $err);
   }
   @if $config and map.has-key($config, scale) {
-    $allowed-scales: (0, -1, -2, -3, -4 -5, minimum, maximum);
+    $allowed-scales: (0, -1, -2, -3, -4, -5, minimum, maximum);
     @if mat.private-validate-allowed-values(map.get($config, scale), $allowed-scales...) {
       @return (
         #{'Expected $config.scale to be one of: #{$allowed-scales}. Got:'}