Extend ExportShapeTool to support the export of image fill content

Add new tool parameter `mode` ("shape" or "fill") to support this

Since the fill's format is not guaranteed to be PNG, include dependency
`sharp` to handle image format conversion

Because the export logic has become more complex, introduce
PenpotUtils.exportImage utility function for minimal code to be
sent to the plugin for execution

Resolves #12

Author: opcode81