Add ability to highlight newly released features in a webview

[pokey]

The problem

There is currently no way for a user to know when we release new features in cursorless. We'd like some way to highlight new features for users.

The solution

We'd like to display release notes in a webview when we release new features. We don't want to do so on every release, especially once we move to continuous delivery (#17), but we should have some mechanism to highlight new features when we want to. We could do something roughly as follows

• [ ] Have a directory of markdown files, one per each feature that we want to highlight
• [ ] Keep track of which feature highlights the user has seen using global storage, making sure to sync across instances
• [ ] Stitch together the highlights they haven't seen and show them in a webview on extension update
• [ ] Figure out a way to include these feature highlight snippets in the docs?
• [ ] Can use these to link to video showing how to use the feature
• [ ] Do this monthly?
• [ ] Also publish this to the website
• [ ] Do this automatically from PRs using some automated system? Maybe https://github.com/uptech/git-cl