Use module build of markmap-lib

If we use the default import then webpack will use the "browser"
bundle which relies on CDN lazy loading of dependencies.

Signed-off-by: Tilman Vatteroth <git@tilmanvatteroth.de>

src/external-types/markmap-lib/index.d.ts

@@ -0,0 +1,9 @@
+/*
+ * SPDX-FileCopyrightText: 2021 The HedgeDoc developers (see AUTHORS file)
+ *
+ * SPDX-License-Identifier: AGPL-3.0-only
+ */
+
+declare module 'markmap-lib/dist/index.esm' {
+  export * from 'markmap-lib'
+}