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?
While isblank() is simple, deleting it would change this pull request from nearly non-breaking to breaking.  (Currently the only behavior change I'm aware of aside from fixing Windows bugs is that isspace() returns `true` rather than `false` for non-breaking space)

Within Base, string.jl contains a couple of calls to isblank().  A quick find-in-files search against my installed packages shows that Jewel.jl uses it, and I suspect other packages do as well.