Distance between the top of the first line to the bottom of the last line in text

Property definitions

gamnit $ TileSetFont :: text_height
	# Distance between the top of the first line to the bottom of the last line in `text`
	fun text_height(text: Text): Numeric
	do
		if text.is_empty then return 0

		var n_lines = text.chars.count('\n')
		return (n_lines+1).mul(height.add(vspace)).sub(vspace)
	end
lib/gamnit/tileset.nit:114,2--121,4