Visual Studio: Support Markdown in Hover response

[chrissimon-au]

Describe the bug
On #28 it was identified that Visual Studio doesn't support markdown. The initialization capability data confirms this and only defines plaintext.

It would be great if Visual Studio also supported markdown (and better emojis).

[chrissimon-au]

Tracked in Visual Studio Developer Community: https://developercommunity.visualstudio.com/t/Support-markdown-in-LSP-textDocumenthov/10712890

And in the VSExtensibility github here: microsoft/VSExtensibility#271