add sortFragmentsOnSync option, makes it possible to avoid unwanted sorting in editing environments like slides.com

Fixes https://github.com/hakimel/reveal.js/issues/3315

Fixes typos

Missing lang attribute

GitHub Workflows security hardening

Gulp livereload: include subfolders to watch for changes in html and md

Add theme 'dracula'

New theme using color palette from [Dracula](https://draculatheme.com/)
dark editor theme available for quite a number of different
applications.

Currently it's only watching for changes to `.html` and `.md` files located in the root of the project. I was frustrated when livereload stopped working for me: turns out it was because I put my content into subfolders.

Signed-off-by: sashashura <93376818+sashashura@users.noreply.github.com>

The lang attribute does not default to English. It defaults to an unknown, which is an accessibility issue.