Update types for then command

[Lakitna]

Closes #40

• Tests
• Type definitions
• Ready for merge

[Lakitna]

@m4dc4p Could you please verify the new types? I have not tests for this stuff, so I would like to make sure that I've not introduced any regressions.

[m4dc4p]

I copied then.d.ts over the existing file in my node_modules. cypress started without complaint, my tests ran, and VS code doesn't report any errors. I think they are good!

Thanks so much!