feat(building-rollup): add main entry point

Author: Westbrook

packages/building-rollup/index.js

@@ -0,0 +1,4 @@
+const createConfig = require('./modern-config.js');
+const createCompatibilityConfig = require('./modern-and-legacy-config.js');
+
+module.exports = { createConfig, createCompatibilityConfig };

packages/building-rollup/package.json

@@ -21,6 +21,7 @@
     "build:ts-babel": "rimraf dist && rollup -c demo/ts-babel/rollup.config.js",
     "prepublishOnly": "../../scripts/insert-header.js"
   },
+  "main": "index.js",
   "keywords": [
     "open-wc",
     "web-components",