Fix slow positionAt impl for markdown references

- 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

extensions/markdown-language-features/src/util/inMemoryDocument.ts

@@ -0,0 +1,46 @@
+/*---------------------------------------------------------------------------------------------
+ *  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 { TextDocument } from 'vscode-languageserver-textdocument';
+import { SkinnyTextDocument, SkinnyTextLine } from '../workspaceContents';
+
+export class InMemoryDocument implements SkinnyTextDocument {
+
+	private readonly _doc: TextDocument;
+
+	private lines: SkinnyTextLine[] | undefined;
+
+	constructor(
+		public readonly uri: vscode.Uri, contents: string,
+		public readonly version = 0,
+	) {
+
+		this._doc = TextDocument.create(uri.toString(), 'markdown', version, contents);
+	}
+
+	get lineCount(): number {
+		return this._doc.lineCount;
+	}
+
+	lineAt(index: any): SkinnyTextLine {
+		if (!this.lines) {
+			this.lines = this._doc.getText().split(/\r?\n/).map(text => ({
+				text,
+				get isEmptyOrWhitespace() { return /^\s*$/.test(text); }
+			}));
+		}
+		return this.lines[index];
+	}
+
+	positionAt(offset: number): vscode.Position {
+		const pos = this._doc.positionAt(offset);
+		return new vscode.Position(pos.line, pos.character);
+	}
+
+	getText(range?: vscode.Range): string {
+		return this._doc.getText(range);
+	}
+}