Update to React 19, consider using esm.sh or other CDN

Problem/Motivation

React 19 is out. For people who want to use it, they need to update the package.json file that's in this project and run the build step to update the files in the js/dist directory.

Or, this project's maintainers can do that, but that then forces React 19 on people who might not be ready to upgrade to it yet.

Which raises the question: why in the age of online build tools like https://esm.sh/ being available, are we including built versions of vendor libraries in this repo?

Steps to reproduce

Proposed resolution

• Change this module to give people a configuration form where they can enter their desired version constraint for React.
• Either on cache clear, or with a dedicated button somewhere, fetch the React library files from https://esm.sh/ and write them to somewhere in public://. Or, perhaps even give people a setting to use directly from esm.sh at runtime.
• Remove package.json and JS files from this repo.

Remaining tasks

Decide if the above proposal is good, or if there are compelling reasons to ship built JS files in this repo.

User interface changes

API changes

Data model changes