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/test/util.ts

@@ -4,7 +4,7 @@
  *--------------------------------------------------------------------------------------------*/
 import * as os from 'os';
 import * as vscode from 'vscode';
-import { InMemoryDocument } from './inMemoryDocument';
+import { InMemoryDocument } from '../util/inMemoryDocument';
 
 export const joinLines = (...args: string[]) =>
 	args.join(os.platform() === 'win32' ? '\r\n' : '\n');