* 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
* Improve markdown Wrapping
Includes a few improvements to the markdown preview
* Use breakword by default to break up very long words
* When `editor.worWrap` is enabled, wrap code blocks inside of the html preview.
* Reverts 41467b74b7 . This change is adding an 20px margin to the left of any html content. I believe the html content itself should be responsible for adding this padding and should normally fill the entire screen.
* Better centers markdown preview content
* Move padding to default webview style
* Remove padding change for now
* Allow markdown preview to scroll past content
Issue #15795
Allows the markdown preview to scroll past its contents, like we do with the editor.
Closes#15795
* Toggle preview scroll beyond using editor.scrollBeyondLastLine
**Bug**
In Markdown preview, code blocks in headers are the wrong font size.
**Fix**
Specify that code blocks in headers should inherit their font-size, instead of brining their own.
