feat: make re2 an optional peer dependency

[aldenquimby]

Fixes #33

Verified as follows:

npm run test

npm uninstall re2

npm run test

As of npm 8+, peer dependencies are automatically installed, so this should not impact consumers by default.

But anyone that wants to explicitly exclude re2 from their build process can now do so without this lib crashing.

[alexturpin]

It would be great to see either this or #35 accepted!

[baptistejamin]

Thank you for the merge request, it has been merged and made available in 1.9.0 :)