Write a new line and indent it, only if pretty_json

Property definitions

json $ JsonSerializer :: new_line_and_indent
	# Write a new line and indent it, only if `pretty_json`
	private fun new_line_and_indent
	do
		if pretty_json then
			stream.write "\n"
			for i in indent_level.times do stream.write "\t"
		end
	end
lib/json/serialization_write.nit:137,2--144,4