:first-of-type and :last-of-type pseudo classes no longer works

[RLesur]

This is a regression introduced by #202

The :last-of-type pseudo class is not recognized by Paged.js (the CSS declaration is deleted).
However, the following workaround works: use :nth-last-of-type(1) instead of :last-of-type (see https://github.com/rstudio/pagedown/commit/893133a4a11990c34bf22fc466cd83c2b5f63408).

Paged.js related issue: https://gitlab.pagedmedia.org/tools/pagedjs/issues/179#note_2443

[RLesur]

Same bug with :first-of-type, see https://community.rstudio.com/t/pagedown-resume-with-one-page-aside-pagedown-0-14/103299