* 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