2582: handle nested DFXP tags in the new editor

Re-wrote dfxpToMarkdown to make it work with nested tags.  Made
dfxpToMarkdown a module-level function, rather than a function of
AmaraDFXPParser.

Made a new test script with tests for dfxpToMarkdown.  Moved the old
one out of the way, since many tests were failing because of errors in
the test code.