`event-lyric-line` should include a word extension object

[rpatters1]

The event-lyric-line object should include an optional wordExtension object that specifies the measure-rhythmic-position of the end of the word extension. We cannot expect consumers to infer this. Indeed, I don't think we want them too. Specifically, MuseScore has to be told the span of a word extension or it won't draw them.

Exporters almost certainly know the extent of word extensions. It would be much better to let them specify it than rely on each importer to figure it out in a different way.

It would be helpful to have a lyrics example with a word extension. This would illustrate the issue.

[mscuthbert]

I think that we need to have a "supports" feature for this -- that the absence of any Word Extension objects does not necessarily mean that the score should have no extensions. Music21 definitely does not calculate these and in musicxml expects renderers to do their best with it. Given that I think there needs to be a value for "No extension" for cases where we want to explicitly show that no extension should be added even when musically it would be normal to have one. (In medieval and Renaissance music, it's common for large passages in an instrumental part to be textless--probably indicating that they're doubled by an instrument and these are instrument only--and it would be incorrect to extend the previous syllable over the textless passage).