Allow defining custom builtins

[edison-moreland]

This is a feature I've found very helpful in my project, SceneEngine.

For my use case, I've got custom user scripts being imported by a runtime script and I need to expose custom builtin functions to that userscript.
I tried creating the builtin as globals, but globals aren't exposed to libraries when they're imported.

[geseq]

Thanks for the PR. I like the idea for this but I'm not keen on breaking the NewVM call for current users for what is likely to be a rarer use case.

Can we instead introduce functional options to pass in the builtins. That way the code remains unbroken for all current users and will also allow future introductions of more options while remaining backwards compatible.