* Adds command to post a message to an html preview
**Bug**
There is currently no easy way to communicate with an html preview document after the preview has been created.
**Fix**
Adds a command called `vscode.htmlPreview.postMessage` to post a message to a visible html preview. This message will only be posted if the target preview is visible.
Inside the preview, the event is recieved using the standard dom event:
* Remove logging
* proto
Continue proto
* clean up rendering
* Gate prototype
* Fix gating
* Remove public command
* Change setting name
* Added current position indicator
* Reveal center
Fixes#4641
Adds basic settings for controlling the markdown preview font. We already support this with custom css, but these settings are similar to what the editor and integrated terminal expose and allow for a few basic customizations. Custom style sheets will override these.
