refactor: update vendor prefix in SCSS

{
  "browsers": [
    "last 2 versions",
    "> 0.2%",
    "not dead"
  ]
}

_sass/addon/syntax.scss

@@ -87,23 +87,21 @@ $code-radius: 6px;
     text-align: right;
     color: var(--highlight-lineno-color);
     -webkit-user-select: none;
-    -khtml-user-select: none;
     -moz-user-select: none;
-    -ms-user-select: none;
     -o-user-select: none;
     user-select: none;
   }
 
   /* set the dollar sign to non-selectable */
   .gp {
+    -webkit-user-select: none;
+    -moz-user-select: none;
     user-select: none;
   }
 } /* .highlight */
 
 code {
   -webkit-hyphens: none;
-  -ms-hyphens: none;
-  -moz-hyphens: none;
   hyphens: none;
 
   &.highlighter-rouge {