server-ts: Port services/export/md

src/types.d.ts

@@ -16,4 +16,12 @@ declare module 'html2plaintext' {
 declare module 'normalize-strings' {
     function normalizeString(string: string): string;
     export = normalizeString;
+}
+
+declare module 'joplin-turndown-plugin-gfm' {
+    import TurndownService = require("turndown");
+    namespace gfm {
+        function gfm(service: TurndownService): void;
+    }
+    export = gfm;
 }