fix(core): fix incorrect `isTauri` return type (#9618)

.changes/api-isTauri-type.md

@@ -0,0 +1,5 @@
+---
+"@tauri-apps/api": "patch:bug"
+---
+
+Fix `isTauri` incorrect return type. 

tooling/api/src/core.ts

@@ -238,8 +238,8 @@ export class Resource {
   }
 }
 
-function isTauri() {
-  return 'isTauri' in window && window.isTauri
+function isTauri(): boolean {
+  return 'isTauri' in window && !!window.isTauri
 }
 
 export type { InvokeArgs, InvokeOptions }