Consolidate focus states into the focus-ring() mixin (#2972)

Step 1 of #2972, output-neutral. `focus-ring()` is now the one
implementation — `outline` + `box-shadow: $focus-ring-box-shadow`, with
`$shadow` (validation colour, range extra layer) and `$outline` (skip the
reset on sibling / `:focus-within` selectors) parameters. Every direct
`box-shadow: $*-focus-box-shadow` consumer calls it; the 14
`$*-focus-box-shadow` variables stay as `!default` aliases.

Compiled CSS byte-identical to `v2-dev`; html-diff, test:scss and lint green.

core/scss/tests/_mixins.test.scss

@@ -149,6 +149,21 @@
       }
     }
   }
+
+  @include it('passes $shadow through and skips the outline reset with $outline: null') {
+    @include assert() {
+      @include output() {
+        .t {
+          @include focus-ring($shadow: 0 0 0 1px red, $outline: null);
+        }
+      }
+      @include expect() {
+        .t {
+          box-shadow: 0 0 0 1px red;
+        }
+      }
+    }
+  }
 }
 
 @include describe('media-print mixin') {