Add quarto excalidraw extension for revealjs (#1286)

Author: parmsam

docs/extensions/listings/revealjs.yml

@@ -139,3 +139,9 @@
   author: "[Sam Parmar](https://github.com/parmsam)"
   description: |
     Adds multiple choice quiz questions to slides.
+
+- name: excalidraw
+  path: https://github.com/parmsam/quarto-excalidraw
+  author: "[Sam Parmar](https://github.com/parmsam)"
+  description: |
+    Adds an empty Excalidraw canvas to your slides for sketching hand-drawn like diagrams.