Generate xml2rfc pdf, txt, html outputs

[ajeanmahoney]

Either individually or all outputs

[JayDaley]

Should any of these be real-time or on demand? For example writing this comment there is a separate "Preview" tab that is an on-demand preview in a new window, whereas VS Code has a live markdown preview in a side-by-side window.

[ajeanmahoney]

For the first version, creating outputs on demand would be sufficient (user selects "Generate all outputs" from a menu, and the outputs are generated and placed in the same directory the XML file is in by default, or in another directory as specified by the user). For a later version, it would be great to have the HTML output visible side-by-side.