Add implicit Suggests 's2' explicitly

[MichaelChirico]

As seen here and a few other places:

plotly.R/tests/testthat/test-ggplot-sf.R

Line 4 in 44499f2

skip_if_not_installed("s2")

[cpsievert]

I would love to freely add suggestions for packages used in testing, but unfortunately CRAN limits the number of suggests, so something like s2 should go under the Config/Needs/check field

[cpsievert]

Thanks!