--format=...  Output format (iife | cjs | umd | esm, no default
                  when not bundling, otherwise default is iife when
                  platform is browser and cjs when platform is node)

UMD format is similar to the IIFE format. Just the module wrapper and setting
the global variable differs. For example:

    // IIFE
    let moduleName = (() => {
      ... bundled code ...
      return exports;
    })();

    // UMD
    (function(root, factory) {
      if (typeof define === 'function' && define.amd) {
        define(factory);
      } else if (typeof module === 'object' && module.exports) {
        module.exports = factory();
      } else {
        root.moduleName = factory();
      }
    }(typeof self !== 'undefined' ? self : this, () => {
      ... bundled code ...
      return exports;
    }));

The module contents is generated with the help of the CJS wrapper, like IIFE.

This change supports only building of standalone applications or libraries. All
dependencies have to be included inside the output bundle or loaded by global
variables. Referring to external dependencies using AMD or CJS is not supported.