Implement GHCi support in HLS

[goldfirere]

Today, I filed haskell/vscode-haskell#450, requesting GHCi support in VSCode, which was then reduced to haskell/haskell-language-server#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.

[jneira]

Many thanks for take care!

[alanz]

See also further discussion on the original issue. haskell/vscode-haskell#450 (comment)