Enable go to definition for markdown links (#148017)

* Enable go to definition for markdown links

* Add markdown definitionProvider tests

extensions/markdown-language-features/src/languageFeatures/definitionProvider.ts

@@ -0,0 +1,21 @@
+/*---------------------------------------------------------------------------------------------
+ *  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 { Disposable } from '../util/dispose';
+import { SkinnyTextDocument } from '../workspaceContents';
+import { MdReferencesProvider } from './references';
+
+export class MdDefinitionProvider extends Disposable implements vscode.DefinitionProvider {
+
+	constructor(private readonly referencesProvider: MdReferencesProvider) {
+		super();
+	}
+
+	async provideDefinition(document: SkinnyTextDocument, position: vscode.Position, token: vscode.CancellationToken): Promise<vscode.Definition | undefined> {
+		const allRefs = await this.referencesProvider.getAllReferencesAtPosition(document, position, token);
+
+		return allRefs.find(ref => ref.kind === 'link' && ref.isDefinition)?.location;
+	}
+}