message: Make build/embed the main start point
diff: {
  "name": "vega-embed",
  - "version": "3.0.0-rc2",
  + "version": "3.0.0-rc3",
  "description": "Publish Vega visualizations as embedded web components.",
  "keywords": [
  "vega",
  }
  ],
  "license": "BSD-3-Clause",
  - "main": "build/vega-embed.js",
  - "index": "build/embed",
  - "jsnext:main": "build/embed",
  + "main": "build/embed",
  "types": "build/embed.d.ts",
  "devDependencies": {
  "@types/d3-selection": "^1.1.0",
  