Resolves https://github.com/elastic/eui-private/issues/136.

Summary

This PR adds a new release CLI for EUI monorepo. It reuses changelog calculation and versioning logic from the old release script and comes with a bunch more safety checks as well as monorepo workspaces awareness.

QA

1. Checkout this PR - gh pr checkout 8308
2. Install dependencies and build release CLI (to be automated) - yarn workspace @elastic/eui-release-cli run build
3. Make sure you're logged out from npm - npm logout and double check with npm whoami
4. Run a custom npm registry locally in a new terminal session - docker run -it --rm --name verdaccio -p 4873:4873 verdaccio/verdaccio
   • The UI is available on http://localhost:4873/
5. Update your local yarnrc to use the local registry for testing. Add the following lines to .yarnrc.yml

   npmRegistryServer: "http://localhost:4873"
   unsafeHttpWhitelist:
     - localhost
     - localhost:4873
   

6. Login to the custom registry - yarn npm login
   1. Confirm the message says Logging in to http://localhost:4873
   2. There's no need to create an account, simply use the same username and password pair (e.g. test / test)
   3. Confirm you're logged in to the right registry with yarn npm whoami
7. Add some changelogs to local packages, e.g., packages/eui/changelogs/upcoming and packages/eslint-plugin/changelogs/upcoming and commit them (temporarily so that the release script can proceed with its init checks)
8. Run yarn release run snapshot --allow-custom
   • This command creates a snapshot release, meaning that it'll publish packages with the snapshot tag and will allow a release from non-main branch.
   • --allow-custom allows releasing code that's different from what's available upstream
9. When asked if you want to proceeed, click Y
10. When asked for an OTP token click Enter, as the local registry has no two-factor authentication configured and OTP is not needed
11. Go to http://localhost:4873 and confirm @elastic/eui and @elastic/eslint-plugin-eui are published to the custom registry
12. Clean up your environment
    1. Logout from the custom registry - yarn npm logout
    2. Remove added lines to .yarnrc.yml
    3. Stop the verdaccio server
    4. Reset your local branch to remote state to remove the temporary commits - git reset --hard