Fix engines.node version in package.json

Not a breaking change, because this module is only compatible with
abstract-level which also has node 12 as minimum.

Author: vweevers

package.json

@@ -36,6 +36,6 @@
     "abstract-level"
   ],
   "engines": {
-    "node": ">=10"
+    "node": ">=12"
   }
 }