Update del requirement from ^3.0.0 to ^4.0.0

Updates the requirements on [del](https://github.com/sindresorhus/del) to permit the latest version.
- [Release notes](https://github.com/sindresorhus/del/releases)
- [Commits](https://github.com/sindresorhus/del/compare/v3.0.0...v4.0.0)

Signed-off-by: dependabot[bot] <support@dependabot.com>

package.json

@@ -39,7 +39,7 @@
     "cpy": "^7.0.1",
     "cross-env": "^5.2.0",
     "cssnano": "^4.1.7",
-    "del": "^3.0.0",
+    "del": "^4.0.0",
     "eslint": "^5.9.0",
     "eslint-config-airbnb-base": "^13.1.0",
     "eslint-config-prettier": "^4.0.0",