Provide a way to parse command line options into BuildOptions

It seems that if you use a build script and want it to also handle command line options, you have to write code to handle them yourself (#3383). It would be nice if there is an API to parse BuildOptions from `process.argv`, so that a build script can just use that and merge them into options parsed to `build`.

My use case is that I want to have a "build" script in package.json, and want to sometimes invoke it with the `--watch` option to use the watch mode.

Start by locating the public BuildOptions API and the existing command-line option parsing entry point. Check how process.argv options such as --watch could be parsed and merged into options passed to build. Done means a documented API supports this build-script use case, with coverage for the requested option behavior.

Written by the indexing model from the issue text.