Convert ANSI-C quoted strings to their value

This makes sense:

> yargs.parse('echo --data "\\n"');
{ _: [ 'echo' ], data: '\\n', '$0': '' }
> yargs.parse("echo --data '\\n'");
{ _: [ 'echo' ], data: '\\n', '$0': '' }

This could be better:

> yargs.parse("echo --data $'\\n'");
{ _: [ 'echo' ], data: "$'\\n'", '$0': '' }

In bash, strings of the form $'string' are a special type of string:

$ echo $'some\\ntext'
some\ntext
$ echo 'some\\ntext'
some\\ntext

See https://www.gnu.org/software/bash/manual/bash.html#ANSI_002dC-Quoting

It would be helpful if yargs returned the raw value for these types of strings as well, instead of making the user parse it themselves.

Start at the yargs.parse entry point and compare its handling of the provided double-quoted, single-quoted, and ANSI-C quoted examples. Use the linked Bash ANSI-C Quoting reference to define the expected conversion, while preserving existing quoted-string behavior. Done means $'...' values are returned in their interpreted form rather than requiring callers to parse them.