Support for walking table components in Lua filters

[jeffvalk]

Currently, components of tables (Cell, Row, TableBody, etc) cannot be used in element filters. The distinction between a typed Pandoc object and an "element component" isn't entirely clear from the Lua filter manual, so I was initially confused when filter functions for Cell did not run.

Making table components behave consistently with other elements was mentioned in https://github.com/jgm/pandoc/issues/7718#issuecomment-996995561. This would be an excellent enhancement. Almost all current "element components" relate to tables. It would be great to have these be first-class citizens in the object model.