Merge this Repo back to check-if-email-exists

[amaury1093]

I'm thinking of putting all the code in this codebase into https://github.com/reacherhq/check-if-email-exists. The reason is two-fold:

• debugging bulk verification logic often requires changes to check-if-email-exists, and tracking changes in 2 repos is hard
• there are often external contributors issues created in one repo when it belongs to the other, which means that people are confused in the 2 repos.

My proposal is that check-if-email-exists will be a monorepo of 3 crates:

• core: the core check-if-email-exists logic, i.e. the existing folder
• cli: a CLI to use the core library with, i.e. this code
• backend: a full-fledged HTTP backend, i.e. all the code in this repo.

[amaury1093]

reacherhq/check-if-email-exists#1138