char predicates using libmojibake

see issue #5939 

Please note from string.jl the following:
```
include("utf8proc.jl")
importall .UTF8proc
```

The include was needed to resolve a bootstrap related error.  Is there a preferred means of dealing with this type of issue?
Do we need this function?   I find it it a bit confusing that we provide both `isspace` and `isblank`, and I suspect that people who need this `isblank` can just as easily write `c==' ' || c=='\t'`.