Inline variable insertion in markdown

[fcollonval]

@chrisjsewell

For example, attachments are a dictionary, whereas user_expressions are, and should be, a list because their calls can have side effects, and so their order is important. So then you are introducing the hack of including the expression index in the key.
Then user_expressions are also not just mimebundles, they can also be error tracebacks. So here you are adding the hack that you give these special mimetype names to distinguish.

Actually this is not what the kernel protocol specified; user_expressions is a dictionary. So indeed if we want to preserve evaluation ordering, it will require changing the Kernel request or add a new on or clarify that keys order matters.

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For now, here are examples of kernel replies:

Nominal case

{
	"user_expressions": {
		"jupyterlab-imarkdown-0": {
			"status": "ok",
			"data": {
				"text/plain": "IntSlider(value=0, layout=Layout(display='inline-flex'))",
				"application/vnd.jupyter.widget-view+json": {
					"version_major": 2,
					"version_minor": 0,
					"model_id": "018244e845c941f886bb20d4c0975320"
				}
			},
			"metadata": {}
		}
	}
}

Error case

{
	"user_expressions": {
		"jupyterlab-imarkdown-0": {
			"status": "error",
			"traceback": [
				"\u001b[0;31mNameError\u001b[0m\u001b[0;31m:\u001b[0m name 'a' is not defined\n"
			],
			"ename": "NameError",
			"evalue": "name 'a' is not defined"
		}
	}
}

So it looks like all the info are there (except the order although that could be enforced by ordering the dictionary keys).