feat: let getTree() append and prepend values to static getTreeUsing array

Author: gp-lnuff

src/SelectTree.php

@@ -550,15 +550,11 @@ public function getTreeUsing(Closure|array $value): static
         return $this;
     }
 
-    public function getTree(): Collection|array
+    public function getTree(): Collection
     {
-        if ($this->getTreeUsing) {
-            return $this->evaluate($this->getTreeUsing);
-        }
-
-        return $this->evaluate($this->buildTree()
+        return Collection::wrap($this->evaluate($this->getTreeUsing)) ?? $this->buildTree()
             ->when($this->prepend, fn (Collection $tree) => $tree->prepend($this->evaluate($this->prepend)))
-            ->when($this->append, fn (Collection $tree) => $tree->push($this->evaluate($this->append)))
+            ->when($this->append, fn (Collection $tree) => $tree->push($this->evaluate($this->append))
         );
     }