add new svelte 4+ detection method

As of Svelte 4, there is a window.__svelte object that can be
used to detect Svelte usage. This commit adds it to the list
of detection methods.

src/technologies/s.json

@@ -6927,6 +6927,9 @@
     "description": "Svelte is a free and open-source front end compiler created by Rich Harris and maintained by the Svelte core team members.",
     "dom": "[class*='svelte-']",
     "icon": "Svelte.svg",
+    "js": {
+      "__svelte": ""
+    },
     "oss": true,
     "website": "https://svelte.dev"
   },