Fix next(::GraphemeIterator) to return SubString

This is consistent with eltype() and therefore collect().
Fixes #9261.

(cherry picked from commit f379e08)
ref #13903

base/unicode/utf8proc.jl

@@ -219,7 +219,7 @@ function next(g::GraphemeIterator, i)
         k = ℓ
         c0 = c
     end
-    return (s[i:j], k)
+    return (SubString(s, i, j), k)
 end
 
 ==(g1::GraphemeIterator, g2::GraphemeIterator) = g1.s == g2.s

test/unicode/utf8proc.jl

@@ -239,7 +239,12 @@ let grphtest = (("b\u0300lahβlahb\u0302láh", ["b\u0300","l","a","h",
             for (s, g) in grphtest
                 s_ = T(normalize_string(s, nf))
                 g_ = map(s -> normalize_string(s, nf), g)
+                # #9261
+                if length(s_) > 0
+                    @test typeof(first(graphemes(s_))) == SubString{typeof(s_)}
+                end
                 grph = collect(graphemes(s_))
+                @test eltype(grph) == SubString{typeof(s_)}
                 @test grph == g_
                 @test length(graphemes(s_)) == length(grph)
             end