Avoid uri parse warnings for markdown uris

Fixes #58566

We only support local file links or a small set of normal schemes, like `http` and `mailto`. Use this to avoid calling `Uri.parse` with scheme-less paths such as `Uri.parse('/images/cat.gif')`

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

@@ -0,0 +1,18 @@
+/*---------------------------------------------------------------------------------------------
+ *  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';
+
+const knownSchemes = ['http:', 'https:', 'file:', 'mailto:'];
+
+export function getUriForLinkWithKnownExternalScheme(
+	link: string,
+): vscode.Uri | undefined {
+	if (knownSchemes.some(knownScheme => link.toLowerCase().startsWith(knownScheme))) {
+		return vscode.Uri.parse(link);
+	}
+
+	return undefined;
+}