fix: removes ?? for node compat (#1574)

We shipped syntax which is beyond our `engine` constraint. `??` is
available in node 14 but 4.x is node 4 and above, while 5.x is node 12
and above.

This just dumbs it back down to `||` for now at least.

Fixes #1573

lib/chai/assertion.js

@@ -66,7 +66,7 @@ module.exports = function (_chai, util) {
     flag(this, 'lockSsfi', lockSsfi);
     flag(this, 'object', obj);
     flag(this, 'message', msg);
-    flag(this, 'eql', config.deepEqual ?? util.eql);
+    flag(this, 'eql', config.deepEqual || util.eql);
 
     return util.proxify(this);
   }