Handle `(pre () (code () ___))` blocks.

The markdown parser pkg previously returned `(pre () ___)` blocks, but
it will be updated to generate `(pre () (code () ___))` blocks. The
latter is what's generated by other markdown parsers; changing to that
facilitates using third-party markdown test suites.

Continue to accept the old form, too, in case someone still has an
older version of the markdown pkg. (Although I'll update the deps to
require the newer version after it's actually released.)