Move core-cli-utils to private/ as a reference implementation (#56922)

Summary:
Pull Request resolved: https://github.com/facebook/react-native/pull/56922

Move `react-native/core-cli-utils` from `packages/` to `private/`, stop publishing it to npm, and reframe it as a reference implementation of React Native CLI tooling.

The package has no known external consumers and is only used internally by `private/helloworld/` and `packages/rn-tester/`. Publishing it to npm creates a maintenance surface for a package that serves no external users.

See https://github.com/react-native-community/discussions-and-proposals/pull/1002.

#### Changes

- Move `packages/core-cli-utils/` to `private/core-cli-utils/`.
- Convert all source files to CJS (require/module.exports) with Flow comment syntax (/*:: */), eliminating the runtime Babel dependency.
 - Remove from the JS build pipeline (`scripts/build/config.js`).
 - Remove the defunct `patchCoreCLIUtilsPackageJSON()` runtime patching from both `helloworld/cli.js` and `rn-tester/cli.js`, and delete both `monorepo.js` files that contained it.
  - Add as `devDependency` to `rn-tester` and `helloworld`.
  - Skip `"*"` version deps in `_prepareHelloWorld()` so they aren't rewritten to the Verdaccio-published version.
  - Remove redundant desktop import ignore entry (`private/**` already covers it).
  - Rewrite README as reference implementation documentation.

Changelog:
[General][Breaking] - The `react-native/core-cli-utils` package is no longer published. It remains available in the React Native repo as a reference implementation.

Reviewed By: cortinico

Differential Revision: D105959855

fbshipit-source-id: 42e439a45273bdeca76029eff306cdf2451308e2