style(theme): do not copy lang when copying text (#4316)

style(theme/vp-doc): let lang unselectable.

Author: sunpm

src/client/theme-default/styles/components/vp-doc.css

@@ -514,6 +514,7 @@
   z-index: 2;
   font-size: 12px;
   font-weight: 500;
+  user-select: none;
   color: var(--vp-code-lang-color);
   transition:
     color 0.4s,