Feature request: Add a function to create `Variations` from edit strings

[MillironX]

Currently there is no public-facing way to construct Variation objects. All of the signatures of Variation() take an Edit, and it's stated in the Discourse post:

Edit will be considered an internal type, and the user is expected to work with Variations. The idea behind that is that by making it difficult to construct Edits without a reference sequence, it’s hard to mess up the reference positions, because you never have to manually manage it.

I propose a function of the signature

Variation(ref::S, edit::AbstractString) where {S<:BioSequence}

which will parse edit, and create a Variation based on ref and the parsed edit.

I'm not quite sure how this will handle the type parameter for T <: BioSymbol within an Edit, or if that will actually be important.

This function will be extremely useful for writing tests.