[DISCUSSION] Refactoring fontification system

[Ihor Radchenko <yantar92@gmail.com>]

Dear all,

Recently, there have been multiple issues related to incorrect
fontification:
- https://list.orgmode.org/orgmode/23707.20428.546749.44853@frac.u-strasbg.fr/
- https://list.orgmode.org/orgmode/87fsujp7mc.fsf@web.de/
- https://list.orgmode.org/orgmode/87czvqxdn9.fsf@gmail.com/
- https://list.orgmode.org/8735nsv9qo.fsf@nicolasgoaziou.fr/T/#me1c44b6e493dd280cca4f042b833c24749ae4fe0

These issues keep appearing because our current fontification code is
based on regexps and only approximates the actual Org grammar elements.
It is largely a legacy from the times when org-element parser was not a
thing.

Maybe it is a time to upgrade the fontification according to our
state-of-art parser?

Instead of fontifying elements individually via regexps, we can leverage
org-element-map, org-element-parse-buffer, org-element-cache, and
jit-lock-mode. Each type of Org element/object can be assigned with a
fontification function accepting a single argument - the element datum.

Also, the fontification code can be move to a separate library.

WDYT?

Best,
Ihor