Bug 1840818: Enable Custom Highlight API in Nightly. r=dom-core,sefeng

Differential Revision: https://phabricator.services.mozilla.com/D182721

modules/libpref/init/StaticPrefList.yaml

@@ -2335,7 +2335,7 @@
 # Disable custom highlight API; implementation pending.
 - name: dom.customHighlightAPI.enabled
   type: RelaxedAtomicBool
-  value: false
+  value: @IS_NIGHTLY_BUILD@
   mirror: always
   rust: true