[Enhancement] NcRichText: `remark-gfm` could be async loaded to reduce initial size

[Antreesy]

rehype-highligh is now async loaded, which reduces the initial component size by ~110kB

Did a quick test, remark-gfm is 40kB and could be extracted as well. Shall we do it?

Originally posted by @Antreesy in #6212 (comment)