Add penguins and penguins_raw to datasets package

[EllaKaye]

This patch adds the penguins and penguins_raw datasets, popularised in the palmerpenguins package, to the datasets package in base R.

The patch includes scripts to prepare the datasets, the data as .rda files, and associated .Rd files.

The reason to include these datasets in base R (rather than to use them though the palmerpenguins package) is to allow us to replace all usage of the iris dataset in examples in base R with comparable examples using penguins instead. That work will be submitted as a separate patch.