Bring back extensions in server package

[wojtekmaj]

Summary

Brings back extensions removed in 114cea2b61ef644d3b794a9b5f48df69f0f5400b, which caused issue #1005.

Test plan

Warning! The infra (particularly Webpack 4) did not let me even install the repository, so this is a "blind fix".

Testing should be fairly simple though if someone is able to:

• Build the package
• Open ESM file from dist directory using node (e.g. node ./dist/esm/loadable-server.esm.mjs)
• Observe if it doesn't crash upon module resolution

[theKashey]

Open ESM file from dist directory using node (e.g. node ./dist/esm/loadable-server.esm.mjs)

import { __SECRET_INTERNALS_DO_NOT_USE_OR_YOU_WILL_BE_FIRED } from '@loadable/component';
import _extends from '@babel/runtime/helpers/esm/extends';
import path from 'path';

Error [ERR_MODULE_NOT_FOUND]: Cannot find module '/Users/akorzunov/dev/github/loaders/loadable-components/node_modules/@babel/runtime/helpers/esm/extends' imported from /Users/akorzunov/dev/github/loaders/loadable-components/packages/server/dist/esm/loadable-server.esm.mjs Did you mean to import @babel/runtime/helpers/esm/extends.js?

Babel reconfiguration is required...

Babel-runtime update is required

[Goodup302]

Hello @theKashey or @wojtekmaj, I don't know if you have this permission, but is it possible to publish version 5.16.5 on npm as soon as possible?