fix(theme-default): make headings anchor non-selectable (#973)

Co-authored-by: meteorlxy <meteor.lxy@foxmail.com>

Author: ramiy

packages/@vuepress/theme-default/src/client/styles/normalize.scss

@@ -129,6 +129,7 @@ a.header-anchor {
   padding-right: 0.23em;
   margin-top: 0.125em;
   opacity: 0;
+  user-select: none;
 
   &:hover {
     text-decoration: none;