Vendor reveal.js 5.2.1 (from 5.1.0)

Contains the upstream 5.2.x feature set, most notably the lightbox
(data-preview-image/data-preview-video opens media full-size in an
overlay), controls: "speaker-only", search API extensions and
speaker-view fixes. The package's local additions are carried over
unchanged: IDS theme (ids.css/ids.scss), extra theme fonts,
chalkboard/customcontrols/menu plugins, and the 1280x720 default
slide size (patched in js/config.js and the dist bundles).

Also fixes a pre-existing fork bug (unrelated to the upgrade): the
slidechanged Highcharts resize hook in default.html now checks that
Highcharts exists, no longer throwing in decks without highcharter.

Docs: README (Rmd + rendered md) gained a short Lightbox section,
NEWS.md notes the library bump.

Verified: examples/ids.html and examples/simple.html re-render and run
error-free through all 13/5 slides (Reveal.VERSION = 5.2.1); lightbox
opens on click and closes with Esc in a dedicated test deck.

Change-Id: I0751129dfb0c9d20b56241377905876314a26011
diff --git a/inst/reveal.js-5.2.1/css/theme/source/moon.scss b/inst/reveal.js-5.2.1/css/theme/source/moon.scss
new file mode 100644
index 0000000..de7d92f
--- /dev/null
+++ b/inst/reveal.js-5.2.1/css/theme/source/moon.scss
@@ -0,0 +1,55 @@
+/**
+ * Solarized Dark theme for reveal.js.
+ * Author: Achim Staebler
+ */
+
+
+// Default mixins and settings -----------------
+@use "sass:color";
+@import "../template/mixins";
+@import "../template/settings";
+// ---------------------------------------------
+
+
+
+// Include theme-specific fonts
+@import url(./fonts/league-gothic/league-gothic.css);
+@import url(https://fonts.googleapis.com/css?family=Lato:400,700,400italic,700italic);
+
+/**
+ * Solarized colors by Ethan Schoonover
+ */
+
+// Solarized colors
+$base03:    #002b36;
+$base02:    #073642;
+$base01:    #586e75;
+$base00:    #657b83;
+$base0:     #839496;
+$base1:     #93a1a1;
+$base2:     #eee8d5;
+$base3:     #fdf6e3;
+$yellow:    #b58900;
+$orange:    #cb4b16;
+$red:       #dc322f;
+$magenta:   #d33682;
+$violet:    #6c71c4;
+$blue:      #268bd2;
+$cyan:      #2aa198;
+$green:     #859900;
+
+// Override theme settings (see ../template/settings.scss)
+$mainColor: $base1;
+$headingColor: $base2;
+$headingTextShadow: none;
+$backgroundColor: $base03;
+$linkColor: $blue;
+$linkColorHover: color.scale( $linkColor, $lightness: 20% );
+$selectionBackgroundColor: $magenta;
+
+// Change text colors against light slide backgrounds
+@include light-bg-text-color(#222);
+
+// Theme template ------------------------------
+@import "../template/theme";
+// ---------------------------------------------