feat(screenshot): add island snap targets (#3881)

port 2.0

(cherry picked from commit 1e31015acb)

quickshell/Modules/DIsland/IslandHostRegistry.qml

@@ -1,32 +0,0 @@
-pragma Singleton
-
-import QtQuick
-import Quickshell
-
-Singleton {
-    property var hosts: ({})
-
-    function key(screenName, barId) {
-        return JSON.stringify([screenName ?? "", barId ?? ""]);
-    }
-
-    function register(key, host) {
-        if (hosts[key] === host)
-            return;
-        const next = Object.assign({}, hosts);
-        next[key] = host;
-        hosts = next;
-    }
-
-    function unregister(key, host) {
-        if (!(key in hosts) || (host && hosts[key] !== host))
-            return;
-        const next = Object.assign({}, hosts);
-        delete next[key];
-        hosts = next;
-    }
-
-    function hostFor(screenName, barId) {
-        return hosts[key(screenName, barId)] ?? null;
-    }
-}