Add support for the seals dataset in ggplot2

[tikkss]

https://ggplot2.tidyverse.org/reference/seals.html
https://github.com/tidyverse/ggplot2/blob/57ba97fa04dadc6fd73db1904e39a09d57a4fcbe/data-raw/seals.csv

License: CC0-1.0

https://community.rstudio.com/t/licenses-for-data-in-tidyverse/129334/7

I actually asked hadley about the datasets, and basically his stance
is that all datasets should be CC0. So, if you want to cite something,
cite the package, or just cite the dataset as CC0.