syntax: Handle \r\n in byte string literals

This ended up passing through the lexer but dying later on in parsing when it
wasn't handled. The strategy taken was to copy the `str_lit` funciton, but adapt
it for bytes.

Closes #16278
As long as you're testing backslash-escaped `\r\n`, you might want to add some whitespace to the line continuation to make sure that's stripped as well.