add config parameter use-psd-smart-object-pixel-scaling

In PS <= 15.0, PSD smart objects were always scaled in pixel space, which led to fuzzy vectors.

In PS >= 15.1, PSD smart objects will now be scaled in a vector-preserving way. Pull request adobe-photoshop/generator-core#224 adds an option to `getPixmap` to force old behavior.

This PR adds a config, called `use-psd-smart-object-pixel-scaling` option to force the old behavior in the assets plugin.

When this PR is merged, the config options wiki page should be updated: https://github.com/adobe-photoshop/generator-assets/wiki/Configuration-Options