[Canvas] Replace the Markdown textarea input with EUI Markdown component

[ryankeairns]

Describe the feature:
Markdown is currently added in the side panel using a fairly standard textarea input. The EUI team is working on a customizable Markdown component that will likely also provide other niceties.

Describe a specific use case for the feature:
Users adding the Text (Markdown) element to their Canvas workpad page, will be able to better identify these enhanced capabilities with a more familiar Markdown editing experience. Further, other non-Markdown capabilities may be included providing a similar experience to that of writing comments on GitHub.

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EUI Markdown component PR: elastic/eui#2597

[elasticmachine]

Pinging @elastic/kibana-canvas (Team:Canvas)

[ThomThomson]

Closing this for the time being because we are not prioritizing Canvas-specific visualization usability.