Add police force area lookup table

## Is your feature request related to a problem? Please describe.
Mostly as documentation for us maintaining the reference list in the screener, giving us a programmatic way to get at it that's documented and reproducible.

## Describe the solution you'd like
Add in data for police force data that replaces the CSV in the screener shiny app repo

## Describe alternatives you've considered
Could just make the eesyscreener package connect to the ONS API directly, but I think doing the documentation / cleaning in dfeR gives us an easier way to manage changes and create a timeseries, and fits more with existing code in dfeR rather than eesyscreener.

## Additional context
Added into the screener shiny app in https://github.com/dfe-analytical-services/dfe-published-data-qa/pull/171