lang/org: major refactor

The motivation for this change was to rethink lang/org's flags. Many of
its former flags represented non-features. Therefore, its flags have
been reduced to five: +dragndrop, +ipython, +pandoc, +gnuplot and
+present. Everything else is included as org-load-hooks and treated as
reasonable defaults.

Other changes:
- Fixes #1502: don't autopair certain pairs when in a math region
- Fixes #1483: broken localleader in org-agenda
- Adds gnuplot support #1108
- Doom's org submodules have been moved into lang/org/contrib/, because
  I expect there will be *many* more to come, and I don't want to
  pollute the moudle's root.

modules/lang/org/contrib/present.el

@@ -0,0 +1,44 @@
+;;; lang/org/contrib/present.el -*- lexical-binding: t; -*-
+
+(defvar +org-present-text-scale 7
+  "The `text-scale-amount' for `org-tree-slide-mode'.")
+
+(after! ox
+  (add-to-list 'org-export-backends 'beamer))
+
+
+;;
+;;; Packages
+
+(def-package! ox-reveal
+  :after ox
+  :init
+  ;; Fix #1127, where ox-reveal adds an errant entry to
+  ;; `org-structure-template-alist'
+  (setq org-reveal-note-key-char nil)
+  :config
+  (setq org-reveal-root "https://cdn.jsdelivr.net/npm/reveal.js@3/"
+        org-reveal-mathjax t))
+
+
+(def-package! org-tree-slide
+  :commands org-tree-slide-mode
+  :config
+  (org-tree-slide-simple-profile)
+  (setq org-tree-slide-skip-outline-level 2
+        org-tree-slide-activate-message " "
+        org-tree-slide-deactivate-message " "
+        org-tree-slide-modeline-display nil)
+
+  (map! :map org-tree-slide-mode-map
+        :n [right] #'org-tree-slide-move-next-tree
+        :n [left]  #'org-tree-slide-move-previous-tree)
+
+  (add-hook! 'org-tree-slide-mode-after-narrow-hook
+    #'(+org-present|detect-slide
+       +org-present|add-overlays
+       org-display-inline-images))
+
+  (add-hook 'org-tree-slide-mode-hook #'+org-present|init-org-tree-window)
+  (advice-add #'org-tree-slide--display-tree-with-narrow
+              :around #'+org-present*narrow-to-subtree))