Replace substitute package

[XhmikosR]

The current substitute package isn't published on npm.

We can either get someone to publish it, or we can just replace its functionality. I think the latter will be simpler.

[Phillip Johnsen]

..or we can just replace its functionality.

Seeing there's about 10 lines of code involved and the code hasn't changed for 5 years, I would also be tempted to copy-paste those lines into this project 🤷‍♂