Enable Web Worker support

[mallardduck]

This commit allows the library to be imported within the context of a web worker. In a web worker, when a script is imported it is outside of the browser's scope and does not have access to window. Data must be passed back and forth between the main and worker thread(s).

Within the Worker, the self should be a sub-type of the WorkerGlobalScope object. The specific sub-type depends on the type of worker - but all workers scope are based on the former. Registering the library to self.cssbeautify allows worker's code to access it via this.cssbeautify.

[mallardduck]

Figured I should provide some examples as clarity to why this is helpful:

Here's a file I call beautify-worker.js:

self.importScripts('/js/cssbeautify.min.js'); // This is the cssbeautify script with my PR.

onmessage = function(e) {
  let inputData = e.data;
  if (null === inputData) {
    postMessage('Please provide an input body');
  } else {
    let output = this.cssbeautify(inputData, {
      indent: '    ',
      openbrace: 'end-of-line',
      autosemicolon: true
    });
    postMessage({ response: output });
  }
}

Then in my main script, or one inline, you can have:

const beautifyWorker = new Worker('/js/beautify-worker.js');
beautifyWorker.onmessage = function(event) {
  // Do some things with the data returned by the worker!
  // With these examples, the beautified CSS is accessible at: event.data.response
};

And then set up a textbox with an event listener that has:

beautifyWorker.postMessage({
  input: data,
});