Rename types in the markdown extension (#152905)

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`

extensions/markdown-language-features/src/test/engine.test.ts

@@ -6,8 +6,8 @@
 import * as assert from 'assert';
 import 'mocha';
 import * as vscode from 'vscode';
-import { createNewMarkdownEngine } from './engine';
 import { InMemoryDocument } from '../util/inMemoryDocument';
+import { createNewMarkdownEngine } from './engine';
 
 
 const testFileName = vscode.Uri.file('test.md');