Merge tag '0.15.0' into develop

Merge branch 'release/0.15.0'

GA: Need to install package itself in order for 'self' vignette engine to be available

GA: modernize + test on more versions

BACKWARD COMPATIBILITY: New squote() failed on R < 3.6.0

The Rgui not loading ~/.Renviron is actually independent of using an .RData file - it simply never process ~/.Renviron unless launched from that folder