Rename uc_is_grapheme_cluster_break() to uc_is_grapheme_break().

Rename uc_is_grapheme_cluster_break() to uc_is_grapheme_break().

This aids consistency, since the "cluster" term is not used elsewhere.

* lib/unigbrk.in.h: Update name.
* lib/unigbrk/u16-grapheme-breaks.c: Update name.
* lib/unigbrk/u16-grapheme-next.c: Update name.
* lib/unigbrk/u16-grapheme-prev.c: Update name.
* lib/unigbrk/u32-grapheme-breaks.c: Update name.
* lib/unigbrk/u32-grapheme-next.c: Update name.
* lib/unigbrk/u32-grapheme-prev.c: Update name.
* lib/unigbrk/u8-grapheme-breaks.c: Update name.
* lib/unigbrk/u8-grapheme-next.c: Update name.
* lib/unigbrk/u8-grapheme-prev.c: Update name.
* lib/unigbrk/uc-is-grapheme-break.c: Update name.
* tests/unigbrk/test-uc-is-grapheme-break.c: Update name.

Suggested by Bruno Haible.