fix: Use let instead of const for let tags on the server

This resolves the issue where we could reassign const variables if the let variable was modified in callback functions.

src/components/let/translate.ts

@@ -54,7 +54,7 @@ export default function translate(tag: t.NodePath<t.MarkoTag>) {
   if (server) {
     file.path.scope.crawl();
     tag.replaceWith(
-      t.variableDeclaration("const", [
+      t.variableDeclaration("let", [
         t.variableDeclarator(tagVar, deepFreeze(file, value)),
       ]),
     );