feat(export/md): rewrite language tag to a more common syntax

spec/services/export/md.spec.ts

@@ -0,0 +1,24 @@
+import markdownExportService from "../../../src/services/export/md.js";
+import { trimIndentation } from "../../support/utils.js";
+
+describe("Markdown export", () => {
+    it("trims language tag for code blocks", () => {
+        const html = trimIndentation`\
+            <p>A diff:</p>
+            <pre><code class="language-text-x-diff">Hello
+            -world
+            +worldy
+            </code></pre>`;
+        const expected = trimIndentation`\
+            A diff:
+
+            \`\`\`diff
+            Hello
+            -world
+            +worldy
+
+            \`\`\``;
+
+        expect(markdownExportService.toMarkdown(html)).toBe(expected);
+    });
+});

src/services/export/md.ts

@@ -8,12 +8,44 @@ let instance: TurndownService | null = null;
 function toMarkdown(content: string) {
     if (instance === null) {
         instance = new TurndownService({ codeBlockStyle: 'fenced' });
+        instance.addRule('fencedCodeBlock', {
+            filter: function (node, options) {
+              return (
+                options.codeBlockStyle === 'fenced' &&
+                node.nodeName === 'PRE' &&
+                node.firstChild !== null &&
+                node.firstChild.nodeName === 'CODE'
+              )
+            },
+          
+            replacement: function (content, node, options) {
+              if (!node.firstChild || !("getAttribute" in node.firstChild) || typeof node.firstChild.getAttribute !== "function") {
+                return content;
+              }
+
+              var className = node.firstChild.getAttribute('class') || ''
+              var language = (className.match(/language-(\S+)/) || [null, ''])[1];
+              language = rewriteLanguageTag(language);
+          
+              return (
+                '\n\n' + options.fence + language + '\n' +
+                node.firstChild.textContent +
+                '\n' + options.fence + '\n\n'
+              )
+            }
+          })
         instance.use(turndownPluginGfm.gfm);
     }
 
     return instance.turndown(content);
 }
 
+function rewriteLanguageTag(source: string) {
+    return source
+        .split("-")
+        .at(-1);
+}
+
 export default {
     toMarkdown
 };