Data::Dumper: useqq implementation for xs

Tests are mainly unchanged, just a "cheat" and a couple of TODOs were
removed.
adjust indentation to match other DD code
handle xs Useqq dumping of strings with an escape followed by a digit

The original patch didn't handle a string like "\x001" correctly, encoding
it as "\01" rather than "\0001".

Added tests for this case and some possible corner cases
[perl #74798] useqq implementation for xs