[Website] Open external links in a new tab/window (#2468)

## Motivation for the change, related issues

Adds `target="_blank"` to external links in Playground web (not CLI).
This ensures external links are loaded in a new tab and not inside the
Playground iframe.

## Implementation details

Adds `a.target = '_blank';` via a `<script>` echoed by an mu-plugin.

## Testing Instructions (or ideally a Blueprint)

Go to wp-admin and click the `WordPress` link in this part of the
footer:

> Thank you for creating with
[WordPress](https://wordpress.org/).Version 6.8.2

cc @fellyph – While looking at your Blueprint I realized this is
something we should solve at the platform level.

cc @JanJakes for reviews

Author: adamziel

packages/playground/remote/src/lib/playground-mu-plugin/0-playground.php

@@ -91,6 +91,61 @@ function networking_disabled() {
 	<?php
 });
 
+/**
+ * Adds target="_blank" to external links when clicked to open them in a new tab.
+ * This prevents users from loading non-Playground pages inside the Playground iframe.
+ */
+function playground_add_target_blank_to_external_links() {
+	// Only run on frontend and admin pages, not during AJAX requests or CLI
+	if (empty($_SERVER['REQUEST_URI']) || wp_doing_ajax() || wp_doing_cron()) {
+		return;
+	}
+	
+	?>
+	<script>
+		function addTargetBlankToExternalLinks() {
+			function addTargetBlank(a) {
+				const url = new URL(a.href, location);
+				if (url.origin !== location.origin) {
+					a.target = '_blank';
+				}
+			}
+
+			// Set target="_blank" for existing external links – this
+			// covers keyboard navigation.
+			document.querySelectorAll('a[href]').forEach(a => {
+				addTargetBlank(a);
+			});
+			
+			// Set target="_blank" for external links when clicked.
+			// This covers links that are added after the page has loaded.
+			document.addEventListener('click', e => {
+				const a = e.target.closest('a[href]');
+				if (!a) return;
+				addTargetBlank(a);
+			});
+
+			// Also handle focus events to cover keyboard navigation on
+			// links that are added after the page has loaded.
+			document.addEventListener('focus', e => {
+				const a = e.target.closest('a[href]');
+				if (!a) return;
+				addTargetBlank(a);
+			}, true);
+		}
+
+		if (document.readyState === 'loading') {
+			document.addEventListener('DOMContentLoaded', addTargetBlankToExternalLinks);
+		} else {
+			addTargetBlankToExternalLinks();
+		}
+	</script>
+
+	<?php
+}
+add_action('wp_head', 'playground_add_target_blank_to_external_links');
+add_action('admin_head', 'playground_add_target_blank_to_external_links');
+
 /**
  * The default WordPress requests transports have been disabled
  * at this point. However, the Requests class requires at least