make BuildOptions immutable

[kevmoo]

It just...hurts

[jakemac53]

I was thinking about exposing this at the top level as well, and splitting it into BuildOptions, WatchOptions, and ServeOptions. Then we could supply a fromArgs constructor as well to support some standard command line args.

This would be a breaking change but would greatly simplify the method signatures and reduce a lot of duplicate named args across the different top level methods.