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);
+    });
+});