FR: option to install a specific version

[DanChaltiel]

Hi,

pkg_install() could have another argument named version which would install the specific version from the archives, similarly to what devtools::install_version would do (but with the nice pak spices).

This would be really nice for the default use-case, i.e. install from CRAN, which is 99% of my own use. I don't know how this would articulate with other syntaxes though.

cc https://github.com/r-lib/pak/issues/360