This introduces a new "script" type of packages that only run a script. This is supposed to be an alternative for optional/experimental packages so we can use third-party package managers to install things.

In this case, the R package manager, though it would presumably be useful for TexLive (instead of src/ext/texlive/texlive-install) and npm. But thats for future tickets...

CC: @jdemeyer @EmmanuelCharpentier

Component: packages: optional

Author: Volker Braun

Branch/Commit: 7316730

Reviewer: Emmanuel Charpentier

Issue created by migration from https://trac.sagemath.org/ticket/19427