fix(theme-default): make sr-only tags unselectable

Author: meteorlxy

packages/@vuepress/theme-default/src/styles/sr-only.styl

@@ -8,4 +8,5 @@
   clip: rect(0, 0, 0, 0);
   white-space: nowrap;
   border-width: 0;
+  user-select: none;
 }