deps: update `cssname` to v5

package.json

@@ -66,7 +66,7 @@
     "chalk": "^4.1.1",
     "cheerio-select-tmp": "^0.1.1",
     "concat-stream": "^2.0.0",
-    "cssnano": "4.1.10",
+    "cssnano": "^5.0.6",
     "deep-freeze": "^0.0.1",
     "disc": "1.3.3",
     "es6-promise": "4.2.8",