$escape() and $unescape() methods

Essentially, Bash allows one to convert ASCII sequences into their corresponding codes if that string is enclosed within single quotes that follow the $.

echo $'this\nstring\thas\tspecial\ncodes'

this
string	has	special
codes

We should support this with an $unescape() method, like so:

echo $unescape('this\nstring\thas\tspecial\ncodes')

As well as a corresponding $escape() method that will do the opposite. We could likely re-use logic from the echo builtin to do this.