[changed] package.browser to package.unpkg

`browser` refers to which APIs a library uses.  The `module` build already uses browser-safe APIs, except for `import` statements.  Any tool that cares about the `browser` field will be able to resolve `import` statements.

CDNs like `unpkg` will serve the JS file directly.  This change ensures they still have a bundled version to serve, without potentially duplicating dependencies if we're bundled downstream.

packages/core/package.json

@@ -3,8 +3,8 @@
   "version": "0.1.0",
   "description": "Makes it easy to add rich, interactive motion to your application.",
   "module": "dist/index.js",
-  "browser": "dist/material-motion.bundle.js",
   "types": "dist/index.d.ts",
+  "unpkg": "dist/material-motion.bundle.js",
   "typescript:main": "src/index.ts",
   "scripts": {
     "lint": "../../node_modules/.bin/tslint -c ../../tslint.json --project tsconfig.json --type-check",