Attempt to support vscode extensions.

[nc6]

VSCode now allows for workspace local extensions. This would be really cool to support in organist, so you can specify e.g. that you would want a certain expression for working on this project:

  vscode = {
    extensions = {
      "nickel" = {
        publisher = "Tweag",
        name = "nickel",
        version = "0.3.0",
        sha256 = "sha256-OntQfxh51B3x92IE4y62bw8csBGukqUzmUJIr/rGioU=",
      },
    },
  },

This PR isn't working yet, but pushing here for help in debugging!