Add Markdown editor field type for editing text

[lane711]

This will be used for editing content such as blog post body content.

We need to find a suitable open source markdown editor such as:

1. https://github.com/marktext/marktext
2. https://mdxeditor.dev/

Developers will need to be able to specify that they want to use the markdown editor while editing certain fields, ie:

export const fields: ApiConfig["fields"] = {
  body: {
    type: "mdxEditor",
  },
}