fix: correctly process empty lines in message markdown (#12057)

Author: Rich-Harris

.changeset/loud-penguins-unite.md

@@ -0,0 +1,5 @@
+---
+'svelte': patch
+---
+
+fix: correctly process empty lines in messages

packages/svelte/scripts/process-messages/index.js

@@ -43,7 +43,7 @@ for (const category of fs.readdirSync('messages')) {
 
 			seen.add(code);
 			messages[category][code] = {
-				messages: sections.map((section) => section.replace(/^> /gm, '')),
+				messages: sections.map((section) => section.replace(/^> /gm, '').replace(/^>\n/gm, '\n')),
 				details
 			};
 		}

packages/svelte/src/internal/client/errors.js

@@ -280,13 +280,13 @@ export function state_prototype_fixed() {
 
 /**
  * Unsafe mutations during Svelte's render or derived phase are not permitted in runes mode. This can lead to unexpected errors and possibly cause infinite loops.
- * >
+ * 
  * If the object is not meant to be reactive, declare it without `$state`
  * @returns {never}
  */
 export function state_unsafe_mutation() {
 	if (DEV) {
-		const error = new Error(`${"state_unsafe_mutation"}\n${"Unsafe mutations during Svelte's render or derived phase are not permitted in runes mode. This can lead to unexpected errors and possibly cause infinite loops.\n>\nIf the object is not meant to be reactive, declare it without `$state`"}`);
+		const error = new Error(`${"state_unsafe_mutation"}\n${"Unsafe mutations during Svelte's render or derived phase are not permitted in runes mode. This can lead to unexpected errors and possibly cause infinite loops.\n\nIf the object is not meant to be reactive, declare it without `$state`"}`);
 
 		error.name = 'Svelte error';
 		throw error;