fix: avoid css tree-shaking for exported Snippet (#18540)

fix #16404 

Currently one file can have multiple exports but only one `cssScopeTo` variable, which is hard coded to `default` inside vite-plugin-svelte.

Therefore set hasGlobal to `true` in case we export a snippet to not have this scoping enabled in v-p-s

---------

Co-authored-by: Simon H <5968653+dummdidumm@users.noreply.github.com>
Co-authored-by: Simon Holthausen <simon.holthausen@vercel.com>

.changeset/three-peas-smoke.md

@@ -0,0 +1,5 @@
+---
+'svelte': patch
+---
+
+fix: avoid css tree-shaking for exported Snippet

packages/svelte/src/compiler/phases/2-analyze/index.js

@@ -832,6 +832,11 @@ export function analyze_component(root, source, options) {
 					} else {
 						e.export_undefined(specifier, name);
 					}
+				} else if (binding.initial?.type === 'SnippetBlock') {
+					// If a snippet is exported, a consumer could only import this named export and not the default export (the component).
+					// In this case we need to set hasGlobal of our output to true so that e.g. vite-plugin-svelte does not tell Vite to
+					// tree-shake the CSS if the default export is not used.
+					analysis.css.has_global = true;
 				}
 			}
 		}

packages/svelte/tests/css/samples/snippet-exported/_config.js

@@ -0,0 +1,5 @@
+import { test } from '../../test';
+
+export default test({
+	hasGlobal: true
+});

packages/svelte/tests/css/samples/snippet-exported/expected.css

@@ -0,0 +1,4 @@
+
+	.inner.svelte-xyz {
+		color: red;
+	}

packages/svelte/tests/css/samples/snippet-exported/input.svelte

@@ -0,0 +1,13 @@
+<script module>
+	export { outer };
+</script>
+
+{#snippet outer()}
+	<p class="inner"></p>
+{/snippet}
+
+<style>
+	.inner {
+		color: red;
+	}
+</style>