User `let` instead `var` for loop index

User `let` instead `var` for loop index to prevent naming collisions with top-level scope.

To reproduce: https://svelte.dev/repl/33152732e43049cfaf171bc6f10fd88f?version=3.13.0

src/compiler/compile/render_dom/wrappers/Element/index.ts

@@ -634,10 +634,10 @@ export default class ElementWrapper extends Wrapper {
 			});
 
 		block.chunks.init.push(b`
-			var ${levels} = [${initial_props}];
+			let ${levels} = [${initial_props}];
 
-			var ${data} = {};
-			for (var #i = 0; #i < ${levels}.length; #i += 1) {
+			let ${data} = {};
+			for (let #i = 0; #i < ${levels}.length; #i += 1) {
 				${data} = @assign(${data}, ${levels}[#i]);
 			}
 		`);
@@ -931,4 +931,4 @@ function to_html(wrappers: Array<ElementWrapper | TextWrapper | TagWrapper>, blo
 			}
 		}
 	});
-}
+}