Upgrade "del" dependency

[madeken]

webpack-dev-server/package.json

Line 45 in 375ab23

"del": "^4.1.1",

Is a bit out of date, and has a bug in which it ends it ends up dragging in @types/node@13.9.3 which can be problematic for typescript users.

[hiroppy]

This will be fixed by #2542.

[alexander-akait]

Done for v4