Add shared monorepo build setup

Summary:
## Context

RFC: Decoupling Flipper from React Native core: react-native-community/discussions-and-proposals#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 and Chokidar. These are available as build commands at the root of the repo:

- `yarn build`
- `yarn clean`
- `yarn watch`

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.

## Current integration points

- `.circleci/config.yml` — Added to `run_yarn` (with project install).
- Root `yarn start` — Convenience for local developers.
- `yarn start` within `packages/rn-tester` — Convenience for local developers.

**Planned**: I will be adding notes on `yarn build` to the following "Contributing" website pages:

- https://reactnative.dev/contributing/overview#contributing-code
- https://reactnative.dev/contributing/how-to-open-a-pull-request#chapter-ii-implementing-your-changes

*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.*

### 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]

Differential Revision: D46759508

fbshipit-source-id: bb99872ffc7ff7c33fd85010a8cfcd6f0508cbff

Author: huntie

.circleci/config.yml

@@ -237,6 +237,9 @@ commands:
           paths:
             - ~/.cache/yarn
           key: << parameters.yarn_base_cache_key >>-{{ arch }}-{{ checksum "yarn.lock" }}
+      - run:
+          name: Build packages
+          command: yarn build
 
   brew_install:
     parameters: