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

@@ -15,7 +15,7 @@
     "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": {