This reverts commit 5055bb7b.

Fixes #32111

Fixes #27797

Fixes #8488

**Bug**
Currently there is no indication in the markdown preview when a `markdown.styles` element fails to load

**Fix**
Show an alert then a stylesheet does not load

Fixes #24808

**bug**
Markdown preview updates when you zoom. The root cause is that previews are updated whenever the config is changed.

**Fix**
Extract preview config to its own well defined object. Only update the preview when the keys we care about in the config change

* Prototype Allowing Extensions to Extend the Builtin Markdown Extension

**Problem**
There have been requests for adding new functionality to the markdown extension preview, such as supporting rendering of math or other syntax in the preview. The only current solution to this is create an extension that provides its own markdown preview. This results in inconsitent behavior with our markdown preview and is not a very scalable approach. We would like to find a way to allow users to add these extensions to our markdown preview without bundling the extensions in the preview itself.

**Fix**
Prototypes a new contribution point that extensions can use to extend the vscode markdown extension. Three types of extensions are possible: adding stypes to the preview, adding scripts to the preview, and extending the markdown it renderer.

My current approach defines the contributed markdown extensions in the package.json using a structure like this:

```
  "contributesTo": {
    "vscode.markdown": {
      "plugins": [
        "./out/math"
      ],
      "scripts": [],
      "styles": [
        "./media/math.css"
      ]
    }
  }
```

We could change the structure here. This design uses a pull model where markdown extensions are looked up by the vscode.markdown extension itself.

The other approach for extension registration would be to use a push model. This would have the vscode.markdown extension export an api that each markdown extension would invoke to register new scripts/styles/plugins. I may switch over to this model but was interested in seeing what a more declarative approach would look like. Let me know if you have any thoughts one way or the other.

The downside of allowing extensions like this is that they can completely change how the markdown preview looks and works. There is no well defined API for restricting what extensions can do like we have with VScode.

* Use extensionDependencies

* Remove example extension

* Added gating and activation event

Adds enhanced security settings for the markdown preview. The new flow disable all scripts within the preview itself. Users can enable scripts on a per workspace basis.

When a markdown document that uses scripts is loaded, a warning is shown inside the document itself. This warning triggers a new security selector quick pick which allows users to enable or disable enahanced security in the workspace.

Fixes #20070

**Bug**
Markdown scroll sync and other sync features do not work for untitled files. The root cause seems to be that the `main.js` markdown file is never loaded

**Fix**
Instead of a using a path without a scheme for `main.js`, use a `file://` path