Add typings

[Artaud]

Add typings and a test as per issue https://github.com/feathersjs-ecosystem/feathers-mailer/issues/29

Dtslint test passes.

[daffl]

Thank you for doing that! I didn't forget about it, I'll just also need to update the module infrastructure (GitHub actions, updated dependencies) for this plugin before making a release.

[AshotN]

@daffl Any chance till will be merged in soon?

[muhammadyusuf-kurbonov]

@daffl When can we expect the merge?

[fratzinger]

I created a typescript version (see #33). Instead of overloading the constructor, I inferred the ReturnType to have typesafe transporter and defaults based on the first argument provided.

[Artaud]

Nice!