* Support Local File Links from the Markdown Preview

Fixes #989

**Bug**

* `file://` links do not work in the markdown preview.
* `./file` and `/file` and `file` links do not work in the markdown preview.

**Fix**
For the first issue, allow `file://` links explicitly.

Add a custom link validator. If the incoming link has no scheme, try to resolve it relative to the current file or workspace. Likes like `/file` are relative to the workspace, while links like `file` or `./file` are relative to the current file.

* Fix reveal of source line on load in markdown preview