Markdown support for Emmet (integration)

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Description

With Emmet, there are a lot of shortcuts and keys to facilitate typing in markdown. Examples:

• Type !+tab and a whole html5 codebase shows up
• Edit various parts at the same time by clicking the cursor in multiple locations while holding the command key (I think it's Crtl on Windows)
• Type cmd+/ to add a comment to a selection
• Tons of keys for various programming languages

And there's much more than that.

Proposal

Make Emmet available for all text fields with markdown support in GitLab

Links / references

• Emmet's source code

Documentation blurb

(Write the start of the documentation of this feature here, include:

1. Why should someone use it; what's the underlying problem.
2. What is the solution.
3. How does someone use this

During implementation, this can then be copied and used as a starter for the documentation.)

cc/ @tauriedavis @victorwu