Backward compatibility with ggplot2 < 3.5.0

[nanxstats]

Using ggsci 3.0.2 (the version that removes scale_name argument) with ggplot2 < 3.5.0, it will complain:

Error in `discrete_scale()`:
! argument "scale_name" is missing, with no default
Backtrace:
 1. ggsci::scale_color_npg()
 2. ggplot2::discrete_scale(...)
 3. ggplot2::ggproto(...)
 4. rlang::list2(...)
Execution halted

There are two options:

1. We could declare the minimum version ggplot2 (>= 3.5.0) in DESCRIPTION, As suggested by Using scale_colour_xxx throws a warning with recent ggplot2 #25 (comment), although that would require users to upgrade ggplot2.

2. Determine the ggplot2 version live and then branch off, to keep it backward compatible. This seem related: shiny:::is_installed().