Fix markdown converter (Promise/await issues)

[floscher]

I've now setup the markdown converter as an abstract class that is implemented differently in client and server:

Can now be called like this and should work again:

fuBlog/client/src/components/MarkDown.vue

Line 39 in 90f24d7

sanitizedHtml.value = await MarkdownConverterClient.Instance.convert(props.markdown ?? "");

The issue was that normally marked.parse(input: string): string returns a string type. But it seems that it suddenly returns a promise if an async MarkedExtension is loaded.