Add additional markdown extensions

This marks the `.litcoffee` file extension as markdown. This is a
markdown format, where indented code blocks can be interpreted as
CoffeeScript.

This also adds the `.mkdn` and `.ron` extensions, which are taken from
https://github.com/sindresorhus/markdown-extensions/blob/main/index.js.

extensions/markdown-basics/package.json

@@ -18,14 +18,17 @@
           "markdown"
         ],
         "extensions": [
+          ".litcoffee",
           ".md",
           ".mkd",
+          ".mkdn",
           ".mdwn",
           ".mdown",
           ".markdown",
           ".markdn",
           ".mdtxt",
           ".mdtext",
+          ".ron",
           ".workbook"
         ],
         "filenamePatterns": [