feat(tree): set Children on Leafs (converts their Node type)

tree/children.go

@@ -33,7 +33,7 @@ func (n NodeChildren) Replace(index int, child Node) NodeChildren {
 	if index < 0 || len(n) < index+1 {
 		return n
 	}
-	return slices.Replace(n, index, index, child)
+	return slices.Replace(n, index, index+1, child)
 }
 
 // Remove removes a child from the list at the given index.

tree/tree.go

@@ -40,6 +40,7 @@ type Node interface {
 	Hidden() bool
 	SetHidden(bool)
 	SetValue(any)
+	Child(...any) *Tree
 }
 
 // Leaf is a node without children.
@@ -78,6 +79,16 @@ func (s *Leaf) SetValue(value any) {
 	}
 }
 
+// Child adds a child to this Tree, converting a Leaf to a Tree in the process.
+func (s *Leaf) Child(children ...any) *Tree {
+	t := &Tree{
+		value:    s.value,
+		hidden:   s.hidden,
+		children: NodeChildren(nil),
+	}
+	return t.Child(children)
+}
+
 // Hidden returns whether a Leaf node is hidden.
 func (s Leaf) Hidden() bool {
 	return s.hidden