This is necessary for Node and similar environments to treat the contents of the esm directory as ES Modules. Otherwise both Node and TypeScript get quite confused.

This is a step in the right direction of resolving #6797. After this change, one should be able to write the following in a TypeScript file:

import ModalDialog from `react-bootstrap/esm/ModalDialog.js`;

function MyComponent() {
  return <ModalDialog></ModalDialog>;
}

Importing e.g. react-bootstrap/ModalDialog doesn't yet work. And the above still doesn't work at runtime because the code in esm/ModalDialog.js isn't actually valid ESM, as it doesn't use extensions on its imports. I'd like to work towards resolving these issues in future PRs, as they're blocking the use of react-bootstrap in my application.