Formalize AddStep

[sagikazarmark]

Is your feature request related to a problem? Please describe.
Currently the first parameter of AddStep allows passing various types:

• string
• []byte
• *regexp.Regex

I think the API here could be improved a bit. It's easier to code when functions have definitive types.

Describe the solution you'd like
I think the most common usage is suite.AddStep(some string step definition), so I would make the first parameter string.

I don't really see why would anyone use a byte slice here. Are there any use cases for that? Maybe we could remove this one.

Finally I would add a suite.AddRegexStep function for accepting a regex instance.

Additional context
I understand this is just polishing, and in a testing use case using reflection doesn't matter that much, but having an easy to read and understand API makes using the library easier.

WDYT?

[bkielbasa]

It makes sense to me. This behaviour is inspired by godog and to be honest I didn't like it as well :)
we can do that!