Remove OS-dependent command invocations

[LPeter1997]

Currently our package.json for the VSC client has the following scripts defined:

  "scripts": {
    "copy-grammar": "(mkdir syntaxes || true) && (cp ../Draco.SyntaxHighlighting/draco.tmLanguage.json ./syntaxes/draco.tmLanguage.json)",
    "vscode:prepublish": "(npm run copy-grammar) && (npm run compile)",
    "compile": "tsc -p ./",
    "watch": "tsc -watch -p ./",
    "lint": "eslint src --ext ts"
  },

Sadly the copying is a Windows-only command, so won't work on other OSes. We can replace with a dev-dependency from NPM or even write a node script inline.