Allow open preview from markdown file title context menu

Part of #37045

extensions/markdown-language-features/package.json

@@ -115,6 +115,13 @@
 					"group": "navigation"
 				}
 			],
+			"editor/title/context": [
+				{
+					"command": "markdown.showPreview",
+					"when": "resourceLangId == markdown",
+					"group": "navigation"
+				}
+			],
 			"commandPalette": [
 				{
 					"command": "markdown.showPreview",

extensions/markdown-language-features/src/commands/showPreview.ts

@@ -80,7 +80,7 @@ export class ShowPreviewCommand implements Command {
 	) { }
 
 	public execute(mainUri?: vscode.Uri, allUris?: vscode.Uri[], previewSettings?: PreviewSettings) {
-		for (const uri of (allUris || [mainUri])) {
+		for (const uri of Array.isArray(allUris) ? allUris : [mainUri]) {
 			showPreview(this.webviewManager, this.telemetryReporter, uri, {
 				sideBySide: false,
 				locked: previewSettings && previewSettings.locked