Returns an array of the grapheme clusters in +self+
(see {Unicode Grapheme Cluster Boundaries}[https://www.unicode.org/reports/tr29/#Grapheme_Cluster_Boundaries]):

  s = "\u0061\u0308-pqr-\u0062\u0308-xyz-\u0063\u0308" # => "ä-pqr-b̈-xyz-c̈"
  s.grapheme_clusters
  # => ["ä", "-", "p", "q", "r", "-", "b̈", "-", "x", "y", "z", "-", "c̈"]