Adds enhanced security settings for the markdown preview. The new flow disable all scripts within the preview itself. Users can enable scripts on a per workspace basis.
When a markdown document that uses scripts is loaded, a warning is shown inside the document itself. This warning triggers a new security selector quick pick which allows users to enable or disable enahanced security in the workspace.
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.
This was causing the output's folder structure to be based on the root of the
vscode project. This bring it in line with the other bundled extensions.
Fixes#15184
