Added `std.utf.decodeBack` which decodes the last UTF code point of given character range.

-------
 import std.utf : decodeBack;

 string text = "サイト";

 assert(decodeBack(text) == 'ト');
 assert(decodeBack(text) == 'イ');
 assert(decodeBack(text) == 'サ');
 assert(text.empty);
-------
