Add vsce script

[Fenix]

I want build it to vsix so that I can install it from local.

I think it's a good idea to add a npm script for packaging like this:

  "scripts": {
    "vscode:prepublish": "webpack --mode production",
    "compile": "webpack --mode development",
    "compile-watch": "webpack --mode development --watch",
    "test-compile": "tsc -p ./",
    "vscode:package": "vsce package"
  },

"devDependencies": {
    // ...other 
    "vsce": "^2.6.7",
    // ...other
  },

[Louis Pilfold]

Thank you