NotebookCellData.mime

## Problem The notebooks api has the concept of markup cells. The intent is that these cells are rendered as html by default, unlike code cells which are displayed as editors by default with their rendered output below them. Currently we only support markdown markup cells. However we'd also like to support other markup languages, including ones that an extension may invent. However there's currently no way to set the mime type of a markup cell, which is needed for a custom renderer to be able to support it ## Proposal Add a new `NotebookCellData.mime` field. If not explicitly provided, this would default to `text/plain` (or `text/markdown` it the cell's `language` is `markdown`) The `mime` field would only be used when rendering the cell. The existing `language` field would still be used to determine which type of editor to display for the cell