`#!import` is broken by Markdown cells in imported `.ipynb` files

[jonsequitur]

The Polyglot Notebooks extension currently writes metadata into Markdown cells indicating that the cell language is the notebook's default (e.g. csharp in the screen shot below). When importing a notebook via #!import or running it via dotnet-repl, this causes the cell to be executed as though it is C#.

I believe that this regression was introduced by #2789:

interactive/src/polyglot-notebooks-vscode-common/src/commands.ts

Lines 307 to 311 in c4bdd27

	// the metadata needs an actual kernel name, not the special-cased 'markdown'
	const kernelNameInMetadata = isMarkdown ? 'csharp' : kernelName;
	const notebookCellMetadata: metadataUtilities.NotebookCellMetadata = {
	kernelName: kernelNameInMetadata,
	};