Add basic markdown link completions

For #140602

Only normal links for now. Will add reference links later. Should support the forms:

- `[](dir/file.md)`
- `[](./dir/file.md)`
- `[](/root-dir/file.md)`
- `[](#header)`
- `[](./dir/file.md#header)`

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

@@ -3,6 +3,31 @@
  *  Licensed under the MIT License. See License.txt in the project root for license information.
  *--------------------------------------------------------------------------------------------*/
 import * as os from 'os';
+import * as vscode from 'vscode';
+import { InMemoryDocument } from './inMemoryDocument';
 
 export const joinLines = (...args: string[]) =>
 	args.join(os.platform() === 'win32' ? '\r\n' : '\n');
+
+export const noopToken = new class implements vscode.CancellationToken {
+	_onCancellationRequestedEmitter = new vscode.EventEmitter<void>();
+	onCancellationRequested = this._onCancellationRequestedEmitter.event;
+
+	get isCancellationRequested() { return false; }
+};
+
+export const CURSOR = '$$CURSOR$$';
+
+export function getCursorPositions(contents: string, doc: InMemoryDocument): vscode.Position[] {
+	let positions: vscode.Position[] = [];
+	let index = 0;
+	let wordLength = 0;
+	while (index !== -1) {
+		index = contents.indexOf(CURSOR, index + wordLength);
+		if (index !== -1) {
+			positions.push(doc.positionAt(index));
+		}
+		wordLength = CURSOR.length;
+	}
+	return positions;
+}