Provide binaries for `haskell-language-server`

Reading through https://github.com/commercialhaskell/stack/blob/d9165e1cfc7a7738145f122f34466d0fa5b9835f/doc/Stack_and_VS_Code.md this sounds complicated and error prone.

Stack already provides GHC binaries and I think if Stack were to provide corresponding `haskell-language-server` binaries for each version of GHC, this could make things much more robust.

In an ideal world a sub command, e.g. `stack lsp` would start a language server, taking care of all the details.

In a second step https://github.com/neovim/nvim-lspconfig, https://github.com/haskell/vscode-haskell, etc. could then be updated to make use of `stack lsp`.

Note that personally I don't have a need for `stack lsp` myself, but I do have a need for the `haskell-language-server` binaries. I have a use case where I use `stack` to install versions of GHC and I'm going to need corresponding `haskell-language-server` binaries.