Use `engine` field to enforce node version

[Aghassi]

Node 16 is now LTS, which means this repo no longer needs to be forward and backwards compatible between zlib and iltorb. In particular, this also solves another problem my org is facing which is that iltorb compilation is a cache miss under bazel because it leaves files with SHAs in them. Enforcing LTS versioning and dropping iltorb from the dep list would solve both of these problems. It would be considered a major, but it's a good move in general.