Add hie.yaml for the Haskell langauge server

Similar to curry-frontend!62 (merged), configuring the language server explicitly makes it easy to work on the frontend, in this case from the parent pakcs repository. Since the language server only inspects the hie.yaml at the root of the workspace, we need one here too.

The language server's implicit configuration unfortunately is not sufficient for this: In the pakcs repository it will default to using the system GHC rather than Stack, presumably since it cannot find a stack.yaml in the root of the repository. This causes errors if the user's system GHC configuration differs in any way from the declared Stack configuration.