Renames in the markdown extension
This renames some types and splits up some files as part of an exploration towards a proper LSP. Changes:
- `SkinnyTextDocument` -> `ITextDocument`
- Moved `ITextDocument` to own file
- `MdWorkspaceContents` -> `IMdWorkspace`
We currently re-compute the same table of contents for markdown files multiple times. This is because multiple language features all need table of contents
With this change, we introduce a new `TableOfContentsProvider` which maintains a cache of the table of contents per file. This provider is then passed into every caller that needs a toc
- Use `vscode-languageserver-textdocument` instead of custom impl
- Reuse `InMemoryDocument` across tests and working code
- Use `SkinnyTextDocument` in more places
- Fixes some test errors that seem to be caused by bad `InMemoryDocument` impl
