Use posix path for markdown links (microsoft#164822)

Fixes microsoft#164782

Author: mjbvz

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

@@ -81,7 +81,7 @@ export function createUriListSnippet(document: vscode.TextDocument, uris: readon
 	const snippet = new vscode.SnippetString();
 	uris.forEach((uri, i) => {
 		const mdPath = dir && dir.scheme === uri.scheme && dir.authority === uri.authority
-			? encodeURI(path.relative(dir.fsPath, uri.fsPath).replace(/\\/g, '/'))
+			? encodeURI(path.posix.relative(dir.path, uri.path))
 			: uri.toString(false);
 
 		const ext = URI.Utils.extname(uri).toLowerCase().replace('.', '');