Support TaskList

[nekoya]

Is your feature request related to a problem? Please describe. When I tried to convert tiptap content containing a TaskList, getMarkdown() returns the contents with redundant line breaks.

- [ ] foo

- [ ] bar

Describe the solution you'd like Pass the tightLists: true option to the MarkdownSerializerState to treat the TaskList as a tight list.

Describe alternatives you've considered Inherit the options of Markdown extension in MarkdownSerializer similar to MarkdownParser. I'm not sure about the benefits on non-tight task lists, but I think the options should be respected.