Port new string.Join overloads

[jamesqo]

Everything from dotnet/coreclr#7942 needs to be ported to this repo. I nearly forgot, so opening a tracking issue for it.

[tonerdo]

@jamesqo @jkotas I see this has been open for quite a while. I'd love to pick it up.

[jkotas]

@tsolarin It's yours. Thank you!

[jamesqo]

@tsolarin @jkotas I forgot to close this issue after fixing it. I have already ported the APIs here: #2270. Sorry for the confusion.