fix: better account for render tags when pruning CSS (#14456)

Fixes #14399

Add a mechanism to connect render tags to snippets to know where to walk when coming across render tags

---------

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

Author: Rich-Harris

.changeset/twelve-squids-wink.md

@@ -0,0 +1,5 @@
+---
+'svelte': patch
+---
+
+fix: better account for render tags when pruning CSS

packages/svelte/src/compiler/phases/1-parse/state/tag.js

@@ -314,7 +314,10 @@ function open(parser) {
 				name
 			},
 			parameters: function_expression.params,
-			body: create_fragment()
+			body: create_fragment(),
+			metadata: {
+				sites: new Set()
+			}
 		});
 		parser.stack.push(block);
 		parser.fragments.push(block.body);
@@ -605,7 +608,8 @@ function special(parser) {
 			metadata: {
 				dynamic: false,
 				args_with_call_expression: new Set(),
-				path: []
+				path: [],
+				snippets: new Set()
 			}
 		});
 	}