Add "Haskell" VSCode Extension Pack

Resharing from hls channel here since it might interesting option to users:

Apparently vscode extension can simultaneously be extension pack Extension Optional Dependency · Issue #6384 · microsoft/vscode · GitHub. Microsoft is doing that with their official python extension vscode-python/gulpfile.js at 66ff9a42990cd538c8363be7218b22548856fa9f · microsoft/vscode-python · GitHub. So there’s no need to publish separate extension or extension pack. If we add extension pack to vscode-haskell, it will install syntax highlighting for all new users of extension when installed (and hopefully when updated for current users as well). If somebody uses other syntax highlighting, extensions from pack can be uninstalled or disabled after initial installation.