fix($theme-default): display header-anchor links when using keyboard navigation (#2699)

Author: ramiy

packages/@vuepress/theme-default/styles/index.styl

@@ -125,6 +125,7 @@ h1, h2, h3, h4, h5, h6
       + p, + pre, + .custom-block
         margin-top 2rem
 
+  &:focus .header-anchor,
   &:hover .header-anchor
     opacity: 1
 
@@ -147,6 +148,7 @@ a.header-anchor
   margin-top 0.125em
   opacity 0
 
+  &:focus,
   &:hover
     text-decoration none