Move {ragg} package from Imports to Suggests

[jrosell]

I’m in a situation where {devtools} imports {pkgdown}, {pkgdown} imports {ragg}, and {ragg} requires the libwebp-devel system package, which I can’t install as a non-admin user.

I suggest using grDevices::png as a fallback when ragg::agg_png is not available, and moving the {ragg} package from Imports to Suggests in {pkgdown}.

Does this make sense to you? Would you be open to a PR where I implement this?

> capabilities("cairo")
cairo 
 TRUE 
> pak::pak("ragg")
                                                                           
→ Will install 1 package.
→ The package (0 B) is cached.
+ ragg   1.5.2 [bld][cmp] + ✔ freetype-devel, ✔ libjpeg-turbo-devel, ✔ libpng-devel, ✔ libtiff-devel, ✖ libwebp-devel
✖ Missing 1 system package. You'll probably need to install it manually:
+ libwebp-devel  - ragg
ℹ No downloads are needed, 1 pkg is cached
✔ Got ragg 1.5.2 (source) (439.05 kB)
ℹ Building ragg 1.5.2
✖ Failed to build ragg 1.5.2 (2.2s)
...