Fix highlight.js files

This is quite a large change but it is mainly for consistency of the file structure of highlight.js.

Some key points to the changes:

* Fixed the update file [after I noticed it was missing commas][1]
* All of the files are now only the compressed version
* Remove 7.3b
* Add versions 7.4 and 8.1

[1]: https://github.com/jsdelivr/jsdelivr/pull/1940#issuecomment-58135308