Add shared monorepo build setup (#38718)

Summary:
Pull Request resolved: https://github.com/facebook/react-native/pull/38718

> NOTE: Replaces https://github.com/facebook/react-native/pull/38240

## Context

RFC: Decoupling Flipper from React Native core: https://github.com/react-native-community/discussions-and-proposals/pull/641

## Changes

To support incoming new React Native packages around debugging (including migrating over [`react-native-community/cli-plugin-metro`](https://github.com/react-native-community/cli/tree/main/packages/cli-plugin-metro)) — which target Node.js and require a build step, this PR adds a minimal shared build setup across the `react-native` monorepo.

The setup is closely inspired/based on the build scripts in Jest, Metro, and React Native CLI — and is a simple set of script wrappers around Babel. These are available as build commands at the root of the repo:

- `yarn build` — Builds all configured packages. Functionally, this:
  - Outputs a `dist/` directory with built files.
  - Rewrites package.json `"exports"` to update every `./src/*` reference to `./dist/*` (source of truth).
- `scripts/build/babel-register.js` — Allows running all Node.js entry points from source, similar to the current setup in [facebook/metro](https://github.com/facebook/metro). (Example entry point file in this PR: `packages/dev-middleware/src/index.js`)

Build configuration (i.e. Babel config) is shared as a set standard across the monorepo, and **packages are opted-in to requiring a build**, configured in `scripts/build.config.js`.

```
const buildConfig /*: BuildConfig */ = {
  // The packages to include for build and their build options
  packages: {
    'dev-middleware': {target: 'node'},
  },
};
```

For now, there is a single `target: 'node'` option — this is necessary as `react-native`, unlike the above other projects, is a repository with packages targeting several runtimes. We may, in future, introduce a build step for other, non-Node, packages — which may be useful for things such as auto-generated TypeScript definitions.

 {F1043312771}

**Differences from the Metro setup**

- References (and compiles out) repo-local `scripts/build/babel-register.js` — removing need for an npm-published dependency.

## Current integration points

- **CircleCI** — `yarn build` is added to the `build_npm_package` and `find_and_publish_bumped_packages` jobs.

**New Node.js package(s) are not load bearing quite yet**: There are not yet any built packages added to the dependencies of `packages/react-native/`, so this will be further tested in a later PR (and is actively being done in an internal commit stack).

### Alternative designs

**Per-package config file**

Replace `scripts/build/config.js` with a package-defined key in in `package.json`, similar to Jest's [`publishConfig`](https://github.com/jestjs/jest/blob/1f019afdcdfc54a6664908bb45f343db4e3d0848/packages/jest-cli/package.json#L87C3-L89C4).

```
"buildConfig": {
  "type": "node"
},
```

This would be the only customisation required, with a single Babel config still standardised. Another option this might receive in future is `enableTypeScriptCodgeen`.

**Rollup**

More sophisticated build tool for Node.js, used by the React codebase (albeit within a custom script setup as well).

**Lerna and Nx**

- Most sophisticated setup enabling caching and optimised cloud runs.
- Probably the most likely thing we'll move towards at a later stage.

Changelog: [Internal]

Reviewed By: NickGerleman

Differential Revision: D47760330

fbshipit-source-id: 38ec94708ce3d9946a197d80885781e9707c5841