Fixes #21511
Fixes #23922

Changes the default markdown toolbar item to open the preview to the side of the current file as requested by #21511

Also updates the single file preview icon slightly to better differentiate it from the diff icon

Two changes:

* Changes the logic from 41467b74 to make the padding customizable. The default 20px padding is preserved, but now preview can customize if they want to use it or not. This allows html preview page background colors to correctly fill the entire frame.

* Remove Writing the `_defaultStyles` to the root of the webview elment. These are only needed inside of the iFrame itself

fixes #24213

fixes #23297

This reverts commit 119d2d4b.

fixes #23297