fix(theming): add aliases for alternate spelling of "grey" (#9903)

Author: jelbourn

src/lib/core/theming/_palette.scss

@@ -630,6 +630,9 @@ $mat-grey: (
   )
 );
 
+// Alias for alternate spelling.
+$mat-gray: $mat-grey;
+
 $mat-blue-grey: (
   50: #eceff1,
   100: #cfd8dc,
@@ -663,6 +666,9 @@ $mat-blue-grey: (
   )
 );
 
+// Alias for alternate spelling.
+$mat-blue-gray: $mat-blue-grey;
+
 
 // Background palette for light themes.
 $mat-light-theme-background: (