Feature request: A togglable version of `data-files`

I'm working on packaging a cabal project which includes quite a few example files which are packaged using `data-files`. I would like to make these files optional instead of being installed by cabal no matter what: tweag/ormolu#682

I understand that `data-files` is generally used for files which are needed after installation, and so having the ability to toggle them with a flag doesn't make much sense. However, a different version of `data-files` with the ability to toggle their installation with a flag would be a nice feature.

Of course, if something with the same functionality already exists, please let me know, but I asked around in `#haskell` and nobody seemed to know of any such feature.

Start by tracing how Cabal handles `data-files` and package flags, then compare the packaging case described in tweag/ormolu#682. Determine the design for making installation of example files optional and verify the behavior in a representative cabal project.

Written by the indexing model from the issue text.