Add reading time, word count about next-mdx

[shadcn]

Fantastic library, I'm really enjoying the simplicity it brings to building an mdx-powered site with relational data.

Thank you.

It would be useful to add reading-time and wordCount parameters to the frontmatter.

I'm using reading-time for a few sites and I agree this would definitely be useful.

Wondering what would the implementation look like.

• We don't need this for all mdx types.
• Make this configurable via next-mdx.json?
• This adds a dependency on the reading-time package.

[shadcn]

@bketelsen A quick update here. I've been working on a Table of Contents module and I think I have an idea how we can solve this with plugins.

I'll post here when I have a working prototype.

Thanks for your help. Really appreciate it.