add as _comments to legacy ast

packages/svelte/src/compiler/legacy.js

@@ -102,7 +102,11 @@ export function convert(source, ast) {
 					instance,
 					module,
 					css: ast.css ? visit(ast.css) : undefined,
-					comments: ast.comments?.length > 0 ? ast.comments : undefined
+					// put it on _comments not comments because the latter is checked by prettier and then fails
+					// if we don't adjust stuff accordingly in our prettier plugin, and so it would be kind of an
+					// indirect breaking change for people updating their Svelte version but not their prettier plugin version.
+					// We can keep it as comments for the modern AST because the modern AST is not used in the plugin yet.
+					_comments: ast.comments?.length > 0 ? ast.comments : undefined
 				};
 			},
 			AnimateDirective(node) {

packages/svelte/tests/parser-legacy/samples/javascript-comments/output.json

@@ -881,7 +881,7 @@
 			"sourceType": "module"
 		}
 	},
-	"comments": [
+	"_comments": [
 		{
 			"type": "Line",
 			"value": " a leading comment",

packages/svelte/tests/parser-legacy/samples/script-comment-only/output.json

@@ -52,7 +52,7 @@
 			]
 		}
 	},
-	"comments": [
+	"_comments": [
 		{
 			"type": "Line",
 			"value": " TODO write some code",