[Export-DbaDbRole] Check for existence of roles

[seanwhitepc]

Apologies if "ideas" is not the right category for this.

Currently, Export-DbaDbRole does not check for the existence of a role before creating it (IF NOT EXISTS...). Similar commands, like Export-DbaUser and Export-DbaLogin, do check for existence ahead of time. It would be nice to have Export-DbaDbRole have consistency with the other commands.

If I find some time, I can work on that, but I'm positive other members of the community can do it faster and more cleanly than I can.