Issue 13802 & 13803 - Improve pretty-print result for the three string types

[Issue 13802](https://issues.dlang.org/show_bug.cgi?id=13802) - Improve pretty-print result for the const(string) type
[Issue 13803](https://issues.dlang.org/show_bug.cgi?id=13803) - Improve pretty-print result for the wstring an dstring types