Update dependency slash to v5 (postcss#442)

Co-authored-by: renovate[bot] <29139614+renovate[bot]@users.noreply.github.com>

Author: renovate[bot]

package.json

@@ -28,7 +28,7 @@
     "postcss-reporter": "^7.0.0",
     "pretty-hrtime": "^1.0.3",
     "read-cache": "^1.0.0",
-    "slash": "^4.0.0",
+    "slash": "^5.0.0",
     "yargs": "^17.0.0"
   },
   "devDependencies": {