Allow SubNav links' focus color to be set (#985)

* set focus styles and allow them to be overriden

* update focus implementation

* add changeset

* github-actions[bot] Regenerated snapshots

* use token

* update changeset

---------

Co-authored-by: joshfarrant <[email protected]>

Author: joshfarrant

.changeset/fair-ladybugs-bathe.md

@@ -0,0 +1,5 @@
+---
+'@primer/react-brand': patch
+---
+
+The focus outline color of `SubNav` links is now set to `var(--brand-color-focus);`

packages/react/src/AnchorNav/AnchorNav.visual.spec.ts-snapshots/Visual-Comparison-AnchorNav-Tablet-viewport-t-c7556--AnchorNav-Regular-view-menu-open-tablet-1-linux.png

@@ -459,6 +459,14 @@
   justify-content: flex-end;
 }
 
+.SubNav__link:focus-visible,
+.SubNav__heading:focus-visible,
+.SubNav__sub-menu-toggle:focus-visible,
+.SubNav__overlay-toggle:focus-visible {
+  outline: var(--brand-borderWidth-thick) solid var(--brand-color-focus);
+  outline-offset: var(--base-size-2);
+}
+
 @media screen and (min-width: 48rem) {
   .SubNav {
     padding: var(--base-size-16) var(--base-size-24);