Add rescript-promise as a peer dependency

[tom-sherman]

This will ensure that users receive a warning (or error in newer npm versions) when they try to install rescript-webapi without also installing @ryyppy/rescript-promise.

This should also go into install instructions but I see the documentation hasn't been updated yet, so this can be done in a seperate PR.

[TheSpyder]

This doesn't seem right? The library is actually only used as a dev dependency (the compiler config ensures that). I don't plan to start using it in the library proper, if we did I'd make it a direct dependency not a peer dependency.

[tom-sherman]

You're absolutely right, not sure what I was thinking 😂