Remove the `String` type constraint in `TreeCharSet`?

[findmyway]

I'd like to customize the color of those characters in TreeCharSet with https://github.com/KristofferC/Crayons.jl . But the type is constrained to String currently. (I remember it was not in prior minor versions). Although I can use the transformed string representation, the textwidth would get the wrong number of text width in that case.