char_superscript incorrectly handles backslashes

Currently, on a superscript grouping ^(like this), backslashes cause an instant end to the group, as though the first backslash seen was a closing paren. This behavior seems undesirable, and most likely unintended.

Example: `^((\^test\))` should result in `<p><sup>(^test)</sup></p>\n`, but actually results in `<p><sup>(\</sup>test))</p>`