Recognize *all* code-chunk terminators

`^@$` is not the only valid terminator. `^@\s\@<=` is a valid terminator as
well. This allows a code chunk to be followed by text immediately,
without a paragraph separator in-between. E.g. like this:

    <<Chunk title>>=
    void some_content() {
        return;
    }
    @ Some text to follow immediately without an empty line in-between
    that gets interpreted as a paragraph separator.

NOTE: Yes, this is part of noweb's actual syntax.