Allow for XML syntax highlighting in markdown cells

[Adzz]

Allows for xml syntax highlighting in markdown cells using monaco.

closes #720

[CLAassistant]

All committers have signed the CLA.

[jonatanklosko]

Thanks @Adzz! Could you add a screenshot for the reference?

Also, feel free to add JSON, if there are no issues with that.

[Adzz]

No problem I need to take another look as I was looking at the wrong thing. This doesn't seem to have done the trick I'll dig further.

[jonatanklosko]

Ahh, I think the tokenizer produces keys that are not covered by our custom theme, I will have a closer look!

[jonatanklosko]

[Adzz]

Ah nice, thank you, sorry I popped out!

One question, when it didn't work I started creating an XML cell is that something you would be open to?

[jonatanklosko]

By an XML cell I assume you mean an input that can be read from? We moved all inputs from Livebook to Kino recently (#714). We may have an input with syntax highlighting at some point, there are just some things for us to explore beforehand.

[josevalim]

@Adzz our general thoughts at this point is to not have cells for things that can be embedded into markdown, with the exception of Elixir. That's because if we want to have a proper code cells, then we need to add all of the related editor features.

But there is a high chance we will add a SQL cell in the future and that may shift the conversation.

At the same time, we are expecting people to write SQL inside Livebook... would anyone write XML?

[Adzz]

... then we need to add all of the related editor features.

Makes sense!

we will add a SQL

Agree, I nearly added SQL to the syntax highlighting too for that reason, totally agree that SQL seems a more natural fit for livebook.

I guess my motivation for adding it was to allow for auto-formatting of the XML - with Floki or similar. Similar to how you can run the elixir formatter on elixir cells.