Interpreter/wrapper for gpar()

[teunbrand]

This PR aims to fix #5866.

Briefly, a new helper function ggpar() is used to wrap grid::gpar() while applying ggplot's interpretation of graphical parameters.

It simplifies several things:

• We don't have to protect against 0-length vectors anymore with len0_null().
• It automatically applies the * .pt modification to lwd.
• It applies the calculation for point stroke and size via stroke and pointsize arguments.

There are two cases where this isn't used:

1. When a default argument is an empty gpar().
2. In sf_grob() which has a mixed interpretations of for example lwd, where in some cases it is linewidth and in other cases it can be stroke.

It is exported but marked as internal function so that extension developers can have access to it as well.

[thomasp85]

LGTM

[davidhodge931]

The gg* prefix is being used by a few extension packages to refer to a wrapper around a lot of ggplot2 code to print a plot as a whole. Such packages include {ggsstatsplot}, {ggpubr}, and {ggblanket}.

I think it'd be cool if possible to keep this distinction as much as possible - so that a function prefixed as gg typically makes a plot (apart from the ggplot_build etc functions). This is good for the intuition of the user, and the use of autocomplete.

Don't know whether you'd consider renaming this function for that reason, @teunbrand ? E.g. gpar2

[teunbrand]

We had already renamed it from ggpar to gg_par precisely because of a namespace clash with {ggpubr}. I'm only willing to consider changing the name if it becomes better, either more self-descriptive or shorter, whereas gpar2() is clearly a worse name for a function.

[davidhodge931]

I think ggplot2::gpar2 is clearer that it is ggplot2's version of gpar. But this stuff is very subjective though