Move toplevel Haskell-centric config files to a new directory

[kostmo]

To build upon #2037, which applied to files that already had a defininitive, pre-existing destination to move to.

This includes:

• fourmolu.yaml
• .stan.toml
• .hlint.yaml
• weeder.toml

See #2044 (comment).

[byorgey]

I'm not sure this is possible, at least in the case of fourmolu. The fourmolu docs say "Fourmolu looks for a fourmolu.yaml file in all parents of the current directory, followed by the XDG config directory", and there doesn't seem to be a flag for specifying an alternative location.