Add support for the economics and economics_long dataset in ggplot2

[tikkss]

https://ggplot2.tidyverse.org/reference/economics.html

• economics: https://github.com/tidyverse/ggplot2/blob/57ba97fa04dadc6fd73db1904e39a09d57a4fcbe/data-raw/economics.csv

• economics_long: unavailable. Maybe generates from economics.csv

  For example economics.csv:

  date,pce,pop,psavert,uempmed,unemploy
  1967-07-01,506.7,198712,12.6,4.5,2944

  Generates economics_long.csv from economics.csv:

  date,variable,value
  1967-07-01,pce,506.7
  1967-07-01,pop,198712
  1967-07-01,psavert,12.6
  1967-07-01,uempmed,4.5
  1967-07-01,unemploy,2944

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.