Recognize more PreTeXt tags

[rogness]

As we work on adding DoenetML to Active Calculus activities, it would be helpful if Doenet would recognize more PreTeXt tag, and (when possible) deal with them gracefully, e.g.:

: recognize and ignore (or treat as section / subsection / similar) : recognize and ignore (or replace with some sort of text about an unknown link, etc.) : typeset the footnote as a parenthetical comment where it appears

: ignore for now, or replace with some sort of alternative text (Duane suggested perhaps the solution is a script to convert these tags to existing Doenet tags. That's also possible; the issue right now is that these are the tags that cause errors and prevent the PreTeXt from rendering as-is when pasted into the Doenet editor.)

[rogness]

<fillin /> adds an underlined blank where a student would fill in a short answer; it's also causing an error in Doenet.