fix: error at compile time when a declaration in a snippet redeclares one of its parameters (#18842)

`{const item = 1}` inside `{#snippet row(item)}` compiles, and the
output declares `item` twice in one function, so it does not parse:

```js
const row = ($$anchor, item = $.noop) => {
	const item = 1;
```

`{let}`, `{@const}` and a nested snippet with the parameter's name do
the same. In `phases/scope.js`, `EachBlock` visits its body's children
in the item's own scope, so a tag that reuses the item name is a
`declaration_duplicate`. `SnippetBlock` calls `context.next({ scope:
child_scope })`, the `Fragment` visitor opens another scope for the
body, and the tag never meets the parameter.

After visiting the body, `SnippetBlock` now reports any name in the
body's scope that `child_scope` also declares. A declaration in a nested
block or element still shadows the parameter.

---------

Co-authored-by: Simon Holthausen <simon.holthausen@vercel.com>

.changeset/snippet-parameter-redeclared.md

@@ -0,0 +1,5 @@
+---
+'svelte': patch
+---
+
+fix: error at compile time when a declaration in a snippet redeclares one of its parameters

packages/svelte/src/compiler/phases/scope.js

@@ -1343,6 +1343,14 @@ export function create_scopes(ast, root, allow_reactive_declarations, parent) {
 			}
 
 			context.next({ scope: child_scope });
+
+			// the parameters and what the body declares end up in the same function
+			const body_scope = /** @type {Scope} */ (scopes.get(node.body));
+			for (const [name, binding] of body_scope.declarations) {
+				if (child_scope.declarations.has(name)) {
+					e.declaration_duplicate(binding.node, name);
+				}
+			}
 		},
 
 		Fragment: (node, context) => {

packages/svelte/tests/validator/samples/declaration-tag-snippet-parameter/errors.json

@@ -0,0 +1,14 @@
+[
+	{
+		"code": "declaration_duplicate",
+		"message": "`item` has already been declared",
+		"start": {
+			"line": 13,
+			"column": 8
+		},
+		"end": {
+			"line": 13,
+			"column": 12
+		}
+	}
+]

packages/svelte/tests/validator/samples/declaration-tag-snippet-parameter/input.svelte

@@ -0,0 +1,17 @@
+<!-- this is ok -->
+{#snippet row1(item)}
+{#if item}
+{const item = 1}
+<p>{item}</p>
+{/if}
+{/snippet}
+
+{@render row1(5)}
+
+<!-- this is not -->
+{#snippet row2(item)}
+	{const item = 1}
+	<p>{item}</p>
+{/snippet}
+
+{@render row2(5)}