Set type in package.json to "module"

This would let Node >=13 to load lit-html as ES modules without any other bundling tools.

Move check-version-tracker file extension to .cjs to continue loading it as cjs.

Fixes #973.

Author: vikerman

check-version-tracker.js renamed to check-version-tracker.cjs

@@ -5,6 +5,7 @@
   "license": "BSD-3-Clause",
   "homepage": "https://lit-element.polymer-project.org/",
   "repository": "Polymer/lit-element",
+  "type": "module",
   "main": "lit-element.js",
   "module": "lit-element.js",
   "directories": {
@@ -31,7 +32,7 @@
     "checksize": "rollup -c ; rm lit-element.bundled.js",
     "format": "find src test | grep '\\.js$\\|\\.ts$' | xargs clang-format --style=file -i",
     "lint": "tslint --project ./",
-    "prepublishOnly": "node check-version-tracker.js && npm run lint && npm test",
+    "prepublishOnly": "node check-version-tracker.cjs && npm run lint && npm test",
     "prepare": "npm run build",
     "regen-package-lock": "rm -rf node_modules package-lock.json; npm install",
     "publish-dev": "npm test && VERSION=${npm_package_version%-*}-dev.`git rev-parse --short HEAD` && npm version --no-git-tag-version $VERSION && npm publish --tag dev",