Add support for insets in the Scalable UI plugin

[rylin8]

This can be a couple of different ways. We can specify the insets in the plugin UI, or directly in Figma by defining some rules such as defining up to four optional rects in each variant named something like "inset-left", "inset-top", etc. If we choose the plugin UI route, this ticket can track the plugin changes. Otherwise, no code changes are needed in this repository. With both methods, the Scalable UI DCF parsing code needs to be updated to read the data.