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
