Format placeholder text in empty blocks to show formatting

Seeing as formatting for text persists when moving between empty blocks (i.e. we can make an empty block italic, leave it, come back, and it will still be formatted as italic) it would be nice if the placeholder text in those blocks was formatted to match the formating in the empty block. In my head, it work the same as how placeholder text for headings is a different size to represent the different block type.