Make Cthulhu a package extension in v1.9?

[giordano]

I'm honestly not sure this is really possible since it doesn't involve dispatch on new datatypes, but I feel like it'd be nicer if code_typed(err; interactive=true) would work out without asking the user to manually install Cthulhu and run using Cthulhu before usage. The package extension would at least remove the latter requirement.