add react-dom/client to rollups externals

[jendrikw]

Fixes an issue where the following code was included in the bundle:

var m = require$$0;

if (process.env.NODE_ENV === 'production') {

  createRoot = m.createRoot;

  m.hydrateRoot;

} else {

  var i = m.__SECRET_INTERNALS_DO_NOT_USE_OR_YOU_WILL_BE_FIRED;

  createRoot = function(c, o) {

    i.usingClientEntryPoint = true;

    try {

      return m.createRoot(c, o);

    } finally {

      i.usingClientEntryPoint = false;

    }

  };

}