Use IE conditional comment placeholders for server island start markers

These placeholders are much less likely to be removed by HTML minifiers,
because they have traditionally been used to change the behavior of
site.

packages/astro/src/runtime/server/render/server-islands.ts

@@ -47,7 +47,7 @@ export function renderServerIsland(
 				}
 			}
 
-			destination.write('<!--server-island-start-->');
+			destination.write('<!--[if astro]>server-island-start<![endif]-->');
 
 			// Render the slots
 			const renderedSlots: Record<string, string> = {};
@@ -88,7 +88,7 @@ if(response.status === 200 && response.headers.get('content-type') === 'text/htm
 	// Swap!
 	while(script.previousSibling &&
 		script.previousSibling.nodeType !== 8 &&
-		script.previousSibling.data !== 'server-island-start') {
+		script.previousSibling.data !== '[if astro]>server-island-start<![endif]') {
 		script.previousSibling.remove();
 	}
 	script.previousSibling?.remove();