Makes a few changes to the markdown extension:
* Enables explicit null checks.
* Enables no implicit any checks.
* Switch to using TS 2.0 lib files.
This change required removing the mocha declaration from the shared `declare.d.ts`. Everything seems to compile fine without it.
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.
* 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
* Add language support for yaml front matter to markdown grammar
* Add option to strip yaml front matter from preview
* Use enum for setting instead of boolean
* Better names
* Fix merge error
