Implement icon reference rendering for plugin tree view tooltips

[colin-grant-work]

What it does

Fixes #10896 by implementing a small plugin for markdownit that adds a rule to replace icon references in plugin tree tooltips with icons.

How to test

1. Open the application, install GitLens, open the Branches view and hover over a branch
2. See this:

Instead of this:

3. Perform the same check on other plugin icon references you're aware of.

Review checklist

• As an author, I have thoroughly tested my changes and carefully followed the review guidelines

Reminder for reviewers

• As a reviewer, I agree to behave in accordance with the review guidelines

[vince-fugnitto]

I confirm that the changes work well, the icons can now be rendered in the tooltips 👍

[colin-grant-work]

@msujew, you've done some work with markdown-it - does this seem like the right way to get rendered icons from it?