Clean up structure of markdown extension (#161148)

- Move things related to the client under `client`
- Remove extra abstractions that are no longer used
- Add MdLanguageClient type

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

@@ -6,7 +6,7 @@
 import * as assert from 'assert';
 import 'mocha';
 import * as vscode from 'vscode';
-import { InMemoryDocument } from '../util/inMemoryDocument';
+import { InMemoryDocument } from '../client/inMemoryDocument';
 import { createNewMarkdownEngine } from './engine';