Adding diff_linesToWords_

[urigoren]

As described here:
https://github.com/google/diff-match-patch/wiki/Line-or-Word-Diffs

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[googlebot]

CLAs look good, thanks!

[emorling]

Will this be merged soon?

[urigoren]

Will this be merged soon?

@NeilFraser

[ghost]

@urigoren would you be able to share an example how this can be used/tested?