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/ast/for.go

@@ -0,0 +1,68 @@
+package ast
+
+import (
+	"fmt"
+
+	"gopkg.in/yaml.v3"
+
+	"github.com/go-task/task/v3/internal/deepcopy"
+)
+
+type For struct {
+	From  string
+	List  []any
+	Var   string
+	Split string
+	As    string
+}
+
+func (f *For) UnmarshalYAML(node *yaml.Node) error {
+	switch node.Kind {
+
+	case yaml.ScalarNode:
+		var from string
+		if err := node.Decode(&from); err != nil {
+			return err
+		}
+		f.From = from
+		return nil
+
+	case yaml.SequenceNode:
+		var list []any
+		if err := node.Decode(&list); err != nil {
+			return err
+		}
+		f.List = list
+		return nil
+
+	case yaml.MappingNode:
+		var forStruct struct {
+			Var   string
+			Split string
+			As    string
+		}
+		if err := node.Decode(&forStruct); err == nil && forStruct.Var != "" {
+			f.Var = forStruct.Var
+			f.Split = forStruct.Split
+			f.As = forStruct.As
+			return nil
+		}
+
+		return fmt.Errorf("yaml: line %d: invalid keys in for", node.Line)
+	}
+
+	return fmt.Errorf("yaml: line %d: cannot unmarshal %s into for", node.Line, node.ShortTag())
+}
+
+func (f *For) DeepCopy() *For {
+	if f == nil {
+		return nil
+	}
+	return &For{
+		From:  f.From,
+		List:  deepcopy.Slice(f.List),
+		Var:   f.Var,
+		Split: f.Split,
+		As:    f.As,
+	}
+}