Returns the selection as Markdown, with lines separated by '\n'. A top-level block selected whole
is copied as its source text, so selecting the whole document gives back the Markdown it was rendered
from. A block only partly selected, at either end of the selection, is rebuilt for its selected part:
its text with its emphasis, code, links and images, inside its heading, list item, quote or table row,
written as normalised Markdown (bold, italic, - bullets). A selection within a
single block copies only its text, with its inline syntax.
public string GetSelectedMarkdown()