replace :before pseudo elements with :marker selector

allows to have different marker based on the level of nesting (as in other themes : disc, square, circle)

fixes #2867

* Fix overlap in overview when config.disableLayout === true

* run gulp js
after commit 9193e5cd5d6d1aa234803b753c6d032b801a8221

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Co-authored-by: Hakim El Hattab <hakim.elhattab@gmail.com>

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