calendar: CSS support for inactive entries

This change adds CSS support for inactive calender entries. Calendar entries can be marked inactive by adding any HTML tag with class inactive to a calendar row. The suggested way to do this is to wrap the date in an inactive span as in:

| [2024-05-21]{.inactive} | pentecost | - |

I was not able to figure out how (or if) to attach CSS classes to the table row itself using Quarto Markdown, thus the workaround with the contained inactive span.

fixes #8 (closed)