[CHANGE] Horizontal Parson problems, backward-incompatible

[Rob Beezer]

There's been a minor adjustment to horizontal Parson problems. Since these are
very new, we have done this in a backward-incompatible way, so the hope is that
this does not impact too many projects or exercises.

Suppose an exercise has blocks that are code, such as an exercise forming an SQL
statement or a regular expression. Then the content needs to be authored
wrapped in an overall "c" element. This is consistent with other new
interactive exercises that involve code - you generally need to use existing
basic PreTeXt markup to indicate text that is code.

You will get warnings in the console, and blocks needing adjustment will be
clearly marked as such in HTML output, so this should be adjusted at your first
opportunity. Changes will roll out as the CLI updates.