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/simple.scss b/inst/reveal.js-5.2.1/css/theme/source/simple.scss
new file mode 100644
index 0000000..7f2d625
--- /dev/null
+++ b/inst/reveal.js-5.2.1/css/theme/source/simple.scss
@@ -0,0 +1,44 @@
+/**
+ * A simple theme for reveal.js presentations, similar
+ * to the default theme. The accent color is darkblue.
+ *
+ * This theme is Copyright (C) 2012 Owen Versteeg, https://github.com/StereotypicalApps. It is MIT licensed.
+ * reveal.js is Copyright (C) 2011-2012 Hakim El Hattab, http://hakim.se
+ */
+
+
+// Default mixins and settings -----------------
+@use "sass:color";
+@import "../template/mixins";
+@import "../template/settings";
+// ---------------------------------------------
+
+
+
+// Include theme-specific fonts
+@import url(https://fonts.googleapis.com/css?family=News+Cycle:400,700);
+@import url(https://fonts.googleapis.com/css?family=Lato:400,700,400italic,700italic);
+
+
+// Override theme settings (see ../template/settings.scss)
+$mainFont: 'Lato', sans-serif;
+$mainColor: #000;
+$headingFont: 'News Cycle', Impact, sans-serif;
+$headingColor: #000;
+$headingTextShadow: none;
+$headingTextTransform: none;
+$backgroundColor: #fff;
+$linkColor: #00008B;
+$linkColorHover: color.scale( $linkColor, $lightness: 20% );
+$selectionBackgroundColor: rgba(0, 0, 0, 0.99);
+
+$overlayElementBgColor: 0, 0, 0;
+$overlayElementFgColor: 240, 240, 240;
+
+// Change text colors against dark slide backgrounds
+@include dark-bg-text-color(#fff);
+
+
+// Theme template ------------------------------
+@import "../template/theme";
+// ---------------------------------------------
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