Unicode: Force-inline isgraphemebreak! (#58674)

When this API was added, this function inlined, which is important,
because the API relies on the allocation of the `Ref` being elided. At
some point (I went back to 1.8) this regressed. For example, it is
currently responsible for substantially all non-Expr allocations in
JuliaParser. Before (parsing all of Base with JuliaParser):
```
│     Memory estimate: 76.93 MiB, allocs estimate: 719922.
```
After:
```
│     Memory estimate: 53.31 MiB, allocs estimate: 156.
```

Also add a test to make sure this doesn't regress again.

(cherry picked from commit d6294ba)

Author: Keno

stdlib/Unicode/test/runtests.jl

@@ -284,6 +284,8 @@ end
     @test_throws BoundsError graphemes("äöüx", 2:5)
     @test_throws BoundsError graphemes("äöüx", 5:5)
     @test_throws ArgumentError graphemes("äöüx", 0:1)
+
+    @test @allocated(length(graphemes("äöüx"))) == 0
 end
 
 @testset "#3721, #6939 up-to-date character widths" begin