Opens the package change log in the system pager so users can read the
version history without leaving their R session. The function reads
NEWS.md from the installed package root (the canonical change log
that R also surfaces via utils::news()). inst/NEWS was
removed in v2.7.1 to eliminate the duplicate-source-of-truth maintenance
hole that left the legacy file frozen at v2.4.0; if any installation
still ships an inst/NEWS, this function falls back to it.