feat: add support for $derived.by type

Author: ota-meshi

src/parser/typescript/analyze/index.ts

@@ -328,6 +328,9 @@ function analyzeRuneVariables(
         appendDeclareFunctionVirtualScripts(globalName, [
           "<T>(expression: T): T",
         ]);
+        appendDeclareNamespaceVirtualScripts(globalName, [
+          "export function by<T>(fn: () => T): T;",
+        ]);
         break;
       }
       case "$effect": {

tests/fixtures/parser/ast/svelte5/docs/runes/02-5-$derived-by-input.svelte

@@ -0,0 +1,14 @@
+<script>
+	let numbers = $state([1, 2, 3]);
+	let total = $derived.by(() => {
+		let total = 0;
+		for (const n of numbers) {
+			total += n;
+		}
+		return total;
+	});
+</script>
+
+<button on:click={() => numbers.push(numbers.length + 1)}>
+	{numbers.join(' + ')} = {total}
+</button>