Set `cwd` to the location of the closest package.json

[hughsk]

This ensures that packages within node_modules that use babelify will resolve babel plugins and presets relative to their own package.json file, instead of resolving plugins and presets from the project root.

This is more consistent with the expected behaviour of browserify transforms, which should work in a dependency without any configuration required by the end user. Previously, developers would have to install all plugins/presets used by babelify within their dependency tree.

Note that this is a breaking change — will likely require a major version bump.

Thanks! :sparkles: