Adds an initial implementation of scroll synrconization from the markdown preview to the editor. When the preview is scrolled, automatically scrolls the editor to reveal the same location.
This required adding a new supported reveal type `AtTop` in our API. This is already supported and used internally, but not really exposed in the public apis (except for the `revealLine` command).
- Basing snippet insertion failure on a new `_codeEditor` null-check.
- Now returns `Thenable<boolean>`.
- Removed vscode.proposed.d.ts copy of the `TextEditor` change.
- Removing empty options interface.
More robust type validation on ext side of insertSnippet.
Position/range check for snippet insertion was inverted.
Adding insertSnippet to vscode.d.ts. (Should it be in vscode.proposed.d.ts?)
Adding extension API tests for insertSnippet.
