Add UMD definitions

Nobody has claimed this yet.

The code assumes that `module` is always available, which is not true (e.g. browser with requireJS). With the following UMD boilerplate it can be much friendlier to use in different environments.

The changes would be straightforward:

1. Remove the `module.exports = punycode;` line at the very end

2. Wrap remaining code like this:
```
;(function (root, factory) {

if (typeof define === 'function' && define.amd) {
define(factory);
} else if (typeof exports === 'object') {
module.exports = factory();
} else {
root.punycode = factory();
}

}(this, function () {

return punycode;
}));
```

Start by locating the source file containing the final module.exports = punycode line and the library code. Apply the requested UMD wrapper, preserving AMD, CommonJS, and browser-global paths, then verify that the library remains usable across those environments.

Written by the indexing model from the issue text.