fix(theme-default): headings anchor should not be selectable (#3063)

Currently, when users select the page content to copy & paste the text, it also selects the headings # anchors. This PR removes the headings anchor-selection by defining the anchors as not selectable (user-select: none;).

Author: ramiy

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

@@ -146,6 +146,7 @@ a.header-anchor
   margin-left -0.87em
   padding-right 0.23em
   margin-top 0.125em
+  user-select none
   opacity 0
 
   &:focus,