Make cross-platform portable

[ghost]

This pull request addresses two bugs exclusive to Windows:

• Breakage during installation caused by oniguruma v7.2.0 addressed by the "Lock first-mate subdependency to v7.3.0" commit
• Path-related errors addressed by the "Update path segment separators to be cross-platform" commit

Details are provided in their respective commit descriptions. Tested on Windows 10 and Debian 9.

[googlebot]

Thanks for your pull request. It looks like this may be your first contribution to a Google open source project (if not, look below for help). Before we can look at your pull request, you'll need to sign a Contributor License Agreement (CLA).

📝 Please visit https://cla.developers.google.com/ to sign.

Once you've signed (or fixed any issues), please reply here with @googlebot I signed it!) and we'll verify it.

---

What to do if you already signed the CLA

Individual signers

• It's possible we don't have your GitHub username or you're using a different email address on your commit. Check your existing CLA data and verify that your email is set on your git commits.

Corporate signers

• Your company has a Point of Contact who decides which employees are authorized to participate. Ask your POC to be added to the group of authorized contributors. If you don't know who your Point of Contact is, direct the Google project maintainer to go/cla#troubleshoot (Public version).
• The email used to register you as an authorized contributor must be the email used for the Git commit. Check your existing CLA data and verify that your email is set on your git commits.
• The email used to register you as an authorized contributor must also be attached to your GitHub account.

ℹ️ Googlers: Go here for more info.

[ghost]

@googlebot I signed it!

[googlebot]

CLAs look good, thanks!

ℹ️ Googlers: Go here for more info.

[sebastianbenz]

Why is this dependency needed?

[ghost]

That's explained in the commit message. I'll paste its contents below for the sake of convenience.

The highlights package depends on a version range of the first-mate package, which currently resolves to v7.4.0. This version of first-mate depends on oniguruma v7.2.0. This version of oniguruma is broken. I am locking first-mate at v7.3.0, which uses the previous functioning version of oniguruma (v7.1.0) in order for npm i to complete.

[alankent]

Hi @sebastianbenz - just a ping on whether you want to accept or reject this PR so we can close it out. Thanks!

[sebastianbenz]

Let's close this for now, as we're going to deprecate the old template build process anyway. The should make sure though that the new version works cross platform.