fix: migrate `$$slots.default` correctly (#13238)

Mentioned in https://github.com/sveltejs/kit/pull/12519#issuecomment-2344826143

packages/svelte/src/compiler/migrate/index.js

@@ -927,6 +927,9 @@ function handle_identifier(node, state, path) {
 	} else if (node.name === '$$slots' && state.analysis.uses_slots) {
 		if (parent?.type === 'MemberExpression') {
 			state.str.update(/** @type {number} */ (node.start), parent.property.start, '');
+			if (parent.property.name === 'default') {
+				state.str.update(parent.property.start, parent.property.end, 'children');
+			}
 		}
 		// else passed as identifier, we don't know what to do here, so let it error
 	}

packages/svelte/tests/migrate/samples/slots/input.svelte

@@ -9,4 +9,6 @@
 	<slot name="bar" />
 {/if}
 
+{#if $$slots.default}foo{/if}
+
 <slot name="dashed-name" />

packages/svelte/tests/migrate/samples/slots/output.svelte

@@ -19,4 +19,6 @@
 	{@render bar?.()}
 {/if}
 
+{#if children}foo{/if}
+
 {@render dashed_name?.()}