refactor: taskfile/ast package (#1450)

* refactor: ast package

* feat: read -> taskfile

* refactor: taskfile.Taskfile -> taskfile.Read

* refactor: move merge function back into taskfile package

* refactor: rename taskfile.go to read.go

taskfile/node_base.go

@@ -0,0 +1,47 @@
+package taskfile
+
+type (
+	NodeOption func(*BaseNode)
+	// BaseNode is a generic node that implements the Parent() and Optional()
+	// methods of the NodeReader interface. It does not implement the Read() method
+	// and it designed to be embedded in other node types so that this boilerplate
+	// code does not need to be repeated.
+	BaseNode struct {
+		parent   Node
+		optional bool
+	}
+)
+
+func NewBaseNode(opts ...NodeOption) *BaseNode {
+	node := &BaseNode{
+		parent:   nil,
+		optional: false,
+	}
+
+	// Apply options
+	for _, opt := range opts {
+		opt(node)
+	}
+
+	return node
+}
+
+func WithParent(parent Node) NodeOption {
+	return func(node *BaseNode) {
+		node.parent = parent
+	}
+}
+
+func (node *BaseNode) Parent() Node {
+	return node.parent
+}
+
+func WithOptional(optional bool) NodeOption {
+	return func(node *BaseNode) {
+		node.optional = optional
+	}
+}
+
+func (node *BaseNode) Optional() bool {
+	return node.optional
+}