Implement actual Preact detection

The detection added in #4056 was looking for [preact-helmet](https://www.npmjs.com/package/preact-helmet), a very specific library that is not a proxy for Preact usage.

src/technologies/p.json

@@ -1263,7 +1263,13 @@
       59
     ],
     "description": "Preact is a JavaScript library that describes itself as a fast 3kB alternative to React with the same ES6 API.",
-    "dom": "meta[data-preact-helmet='true']",
+    "dom": {
+      "#app, .app, #root, .root, body, body > *, body > * > *, body > * > * > *": {
+        "properties": {
+          "__k": ""
+        },
+      }
+    },
     "icon": "Preact.svg",
     "oss": true,
     "website": "https://preactjs.com"
@@ -1909,4 +1915,4 @@
     },
     "website": "http://punbb.informer.com"
   }
-}
+}