chore(tooling): Switch from `prepublish` to `prepare` (#649)

As described in the [NPM documentation](https://docs.npmjs.com/misc/scripts#prepublish-and-prepare), the `prepublish` script is deprecated for this use-case. Instead, `prepare` should be used for essential build tasks.

In practice, this makes it possible to depend on `react-native-webview` via git. If I contribute a new feature, I can use the feature right away by depending on its git branch, without waiting for upstream to publish, or even approve my pull request. This is great for experimentation.

package.json

     "ci:publish": "yarn semantic-release",
     "lint": "yarn tsc --noEmit && yarn eslint ./src --ext .ts,.tsx",
     "build": "yarn tsc",
-    "prepack": "yarn build",
+    "prepare": "yarn build",
     "test": "yarn jest"
   },
   "peerDependencies": {