Update dependency slash to v4

[renovate[bot]]

This PR contains the following updates:

Package	Change	Age	Adoption	Passing	Confidence
slash	^3.0.0 -> ^4.0.0

---

Release Notes

sindresorhus/slash

### [`v4.0.0`](https://togithub.com/sindresorhus/slash/releases/v4.0.0) [Compare Source](https://togithub.com/sindresorhus/slash/compare/v3.0.0...v4.0.0) ##### Breaking - Require Node.js 12 [`998dcc8`](https://togithub.com/sindresorhus/slash/commit/998dcc8) - This package is now pure ESM. Please [read this](https://gist.github.com/sindresorhus/a39789f98801d908bbc7ff3ecc99d99c).

---

Configuration

📅 Schedule: At any time (no schedule defined).

🚦 Automerge: Disabled by config. Please merge this manually once you are satisfied.

♻ Rebasing: Whenever PR becomes conflicted, or you tick the rebase/retry checkbox.

🔕 Ignore: Close this PR and you won't be reminded about this update again.

---

• [ ] If you want to rebase/retry this PR, check this box.

---

This PR has been generated by WhiteSource Renovate. View repository job log here.

[RyanZim]

We still support Node 10