Remove `regenerator-runtime` dependency

[Balearica]

I am not sure exactly why regenerator-runtime is used or what it does. Per a quick Google search it appears to replace generator functions with more verbose code that runs on all platforms. However, generator functions have been available on every major platform since late 2016, so it is unclear what this does now.

https://facebook.github.io/regenerator/

[Note: This is a TODO for me to implement, and is mostly a research task. Unless you can explain what the regenerator-runtime function is, why it would be needed historically, and why it can be removed, please do not open a PR that just cuts it out.]