Expose build plugin

[alanz]

The jsonstdio implementation has a build plugin, which is not currently exposed via LSP.

Should we expose it, and if so how?

Perhaps based on the Build System Protocol BSP as proposed by the Scala people.

cc @arrowd

[lukel97]

The BSP looks promising: The specification seems to fit Cabal projects well, and the underlying JSON RPC protocol remains the same so we should be able to reuse some stuff. Although I think it would still probably be worthwhile to start a new package for it, such as haskell-bsp or the like.

And in the meantime, as long as we also expose LSP commands for build commands, we can integrate them into vscode-hie-server ourselves