Implement Masks handling

Currently there is no special handling of masks in Figma plugin - we export metadata as if no masks were applied.

For now we should provide a basic solution that enables baking of masks and items influenced by masks into assets during export.

There should be a setting that allows user to pick between mask export mode. Similar to effects support, we should at first provide "ignore" and "rasterize" options, but research and add dynamic masks generation eventually later.
There should be a Node Info entry about a mask and applied mask on currently selected item.

It's possible in Figma to have multiple masks within one parent, and we have to generate an image per mask.
It means, that we need to be able to understand where a mask coverage starts and ends, separating them into a cluster of nodes to be rasterized.