Replace .npmignore with package.json "files" option. (#5245)

This effectively removes the following from the NPM packages:
* bower_components
* dist
* CONTRIBUTING.md
* gen-tsd.json
* gulpfile.js
* wct.conf.json

Fixes #5244

Author: coreyfarrell

.npmignore

@@ -64,5 +64,12 @@
   },
   "dependencies": {
     "@webcomponents/shadycss": "^1.2.0"
-  }
+  },
+  "files": [
+    "externs",
+    "lib",
+    "index.html",
+    "polymer-*.js",
+    "manifest.json"
+  ]
 }