Determines whether the character pointed by this is part of a natural-language word (as opposed to say inside some whitespace).
Word breaks are determined by Pango and should be correct for nearly any language (if not, the correct fix would be to the Pango word break algorithms).
Note that if starts_word returns true, then this function returns true too, since this points to the first character of the word.
| this |
a TextIter |
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true if this is inside a word |