Use grapheme clusters in unicodeJS.TextString

unicodejs.js:
* add splitClusters(text) and splitCharacters(text) methods

unicodejs.textstring.js:
* change internal representation from a char string to a list of grapheme
  clusters

unicodejs.wordbreak.js:
* change getGroup to work on the first character of a grapheme cluster

ve.js:
* Use new unicodejs.splitClusters function

Bug: 48975
Change-Id: I202b98199d2780534d1e02519b72579ba796f08f
Merge "Use grapheme clusters in unicodeJS.TextString"