Make Cthulhu a package extension in v1.9?

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.

Start by tracing the `code_typed(err; interactive=true)` entry point and reviewing how Julia package extensions are configured. Determine whether Cthulhu can be integrated so the interactive path works after installing Cthulhu without a manual `using Cthulhu`; done means the feasibility and required extension behavior are established.

Written by the indexing model from the issue text.