feat(markdown): add render command (fixes #75612)

This adds a command which renders the provided document, or the active
editor if one is provided. Following the pattern of some of the preview
commands, it returned `undefined` if there's no document provided and
no active text editor. Otherwise, seems to work...

```ts
const html = await vscode.commands.executeCommand<string>('markdown.render');
```

A way to render arbitrary strings in addition to documents may be useful at
some point in the future. However, I didn't implement that here as that'd
require some refactoring of the markdown engine. If we're interested though
I could certainly give that a shot.

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

@@ -0,0 +1,30 @@
+/*---------------------------------------------------------------------------------------------
+ *  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 { Command } from '../commandManager';
+import { MarkdownEngine } from '../markdownEngine';
+import { SkinnyTextDocument } from '../tableOfContentsProvider';
+
+export class RenderDocument implements Command {
+	public readonly id = 'markdown.render';
+
+	public constructor(
+		private readonly engine: MarkdownEngine
+	) { }
+
+	public async execute(document?: SkinnyTextDocument): Promise<string | undefined> {
+		if (!document) {
+			if (!vscode.window.activeTextEditor) {
+				return;
+			}
+
+			document = vscode.window.activeTextEditor.document;
+		}
+
+		return this.engine.render(document);
+	}
+}