refactor: use new method of accessing zlib constants

"Accessing the constants directly from the module is currently still possible but is deprecated."
https://nodejs.org/api/zlib.html#zlib_zlib_constants

lib/filter.js

@@ -115,7 +115,7 @@ function logicGzip () {
       assetPath.on('end', () => {
         if (assetTxt.length) {
           // gzip compress using highest level
-          zlib.gzip(assetTxt, { level: zlib.Z_BEST_COMPRESSION }, (err, Input) => {
+          zlib.gzip(assetTxt, { level: zlib.constants.Z_BEST_COMPRESSION }, (err, Input) => {
             if (!err) {
               // Save the compressed file to .gz
               route.set(path + '.gz', Input)