Returns true if iter points to the start of the paragraph delimiter
characters for a line (delimiters will be either a newline, a carriage
return, a carriage return followed by a newline, or a Unicode paragraph
separator character). Note that an iterator pointing to the \n of a
\r\n pair will not be counted as the end of a line, the line ends before
the \r. The end iterator is considered to be at the end of a line,
even though there are no paragraph delimiter chars there.