Finalize product icon theme contributions

[aeschli]

Product icons themes allow theme authors to customize the icons used in VS Code's built-in views: all icons except file icons (covered by file icon themes) and icons contributed by extensions.

The API consists of

• a new contribution point: productIconThemes

  "contributes": {
    "productIconThemes": [
      {
        "id": "elegant",
        "label": "Elegant Icon Theme",
        "path": "./producticons/elegant-product-icon-theme.json"
      }
    ]
  }
}

• the product icon definition file schema

Both APIs have been in preview for several milestones. A sample is here: https://github.com/microsoft/vscode-extension-samples/tree/master/product-icon-theme-sample