Implement GHCi support in HLS

Today, I filed https://github.com/haskell/vscode-haskell/issues/450, requesting GHCi support in VSCode, which was then reduced to https://github.com/haskell/haskell-language-server/issues/477, requesting GHCi support in HLS. This latter ticket is 11 months old, with some nice design thoughts, but no known implementation work. As @alanz says: "This is the last major piece missing from our IDE offering."

I was going to write an HF proposal that we drum up some action to nab this ticket, but the proposal process itself is in flux. So I will content myself with filing this Issue, which, after the process is settled, I (or anyone else!) will turn into a proper proposal.