Codegen: Add prepublish script to build Flow files (#28827)

Summary:
*This is a follow-up to https://github.com/facebook/react-native/issues/28645, redone using a build script based off of Metro's build script instead of using `flow-remove-types` and `flow-copy-source`.*

This pull request adds a build step to `react-native-codegen` that builds the Flow-annotated JS files so that users of the NPM module `react-native-codegen` do not need to use require hooks to be able to import it.

A new build script, `scripts/build.js` is added that builds every JS file in `src/` into a `lib/` folder, and also copies over the original Flow annotated files to `lib/` with a `.js.flow` extension, so users of `react-native-codegen` can still typecheck against it using Flow. The shell scripts in `src` are also copied over. It is based off of the [build script from Metro](https://github.com/facebook/metro/blob/00867816eb9b2f69c8af9cebb523e9e4d280671a/scripts/build.js)

## Changelog

[General] [Added] - Codegen: Add prepublish script to build Flow files
Pull Request resolved: https://github.com/facebook/react-native/pull/28827

Test Plan:
I am able to make use of the Codegen scripts without needing to use the `flow-node` CLI or the `flow-remove-types/register`
require hook.

Reviewed By: cpojer

Differential Revision: D21412173

Pulled By: hramos

fbshipit-source-id: 26ae67cdd04652ca4700a069a234a25558773cb1