add two unsupported self defined unit for later dynamic px2dp sizing

[5468sun]

Im using self defined unit pxdp (pixel to Density-independent pixels) and pxfz (pixel to font size) to do dynamic sizing later. but throw exception when packaging. I think we should add this two units or better remove the restrain.

[jacobp100]

Thanks for the PR!

In CSS, pixels are density independent - so you might be able to just use that directly!

I understand we currently support units not available in RN, but I think adding units not available in CSS is possibly a step too far for this project at least. Although you're more than welcome to fork if you need this for your project.