Rename Option.getOrDefault to valueOrDefault, and ArgParser.getDefault to default

[Nate Bosch]

I wish we had caught this earlier, normally I would like to have a period of time with a @deprecated annotation on the old one to let folks migrate to the new one.

I think we can probably do that here too and we'll just be stuck with the old aliases until the next major version.

I'm almost tempted to do the breaking change, but I'd rather not prevent packages from having a wide constraint across the major version bump and I don't think we have the time to roll out a non-breaking version of the package that adds the deprecations.

cc @munificent