Add C escape sequences

[czkz]

Some C escape sequences are not supported in Janet strings, namely:

• \'
• \?
• \a
• \b

Full list at https://en.cppreference.com/w/c/language/escape. Supporting them could be useful to copy-paste strings from other languages (and I need \b).

[ml-2]

You probably already know this, but you can use \x08 instead of \b if you need it right now in Janet.

[bakpakin]

We don't really need things like \? and \' but I suppose they don't hurt for copy pasting.