fix(material-experimental/button): buttons not outlined in high contrast mode (#16410)

Fixes the MDC-based buttons blending in with the background in high contrast mode. In general MDC doesn't include high contrast styling, whereas our current components do.

Author: crisbeto

src/material-experimental/mdc-button/button.scss

@@ -1,5 +1,6 @@
 @import '@material/button/mixins';
 @import '../mdc-helpers/mdc-helpers';
+@import '../../cdk/a11y/a11y';
 
 @include mdc-button-without-ripple($query: $mat-base-styles-query);
 
@@ -12,4 +13,12 @@ $_mat-button-ripple-opacity: 0.1;
     opacity: $_mat-button-ripple-opacity;
     background-color: currentColor;
   }
+
+  // Add an outline to make buttons more visible in high contrast mode. Stroked buttons
+  // don't need a special look in high-contrast mode, because those already have an outline.
+  @include cdk-high-contrast {
+    &:not(.mdc-button--outlined) {
+      outline: solid 1px;
+    }
+  }
 }