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/client/inMemoryDocument.ts

@@ -0,0 +1,20 @@
+/*---------------------------------------------------------------------------------------------
+ *  Copyright (c) Microsoft Corporation. All rights reserved.
+ *  Licensed under the MIT License. See License.txt in the project root for license information.
+ *--------------------------------------------------------------------------------------------*/
+
+import * as vscode from 'vscode';
+import { ITextDocument } from '../types/textDocument';
+
+export class InMemoryDocument implements ITextDocument {
+
+	constructor(
+		public readonly uri: vscode.Uri,
+		private readonly contents: string,
+		public readonly version = 0,
+	) { }
+
+	getText(): string {
+		return this.contents;
+	}
+}