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/tasks.go

@@ -0,0 +1,54 @@
+package ast
+
+import (
+	"fmt"
+
+	"gopkg.in/yaml.v3"
+
+	"github.com/go-task/task/v3/internal/orderedmap"
+)
+
+// Tasks represents a group of tasks
+type Tasks struct {
+	orderedmap.OrderedMap[string, *Task]
+}
+
+func (t *Tasks) UnmarshalYAML(node *yaml.Node) error {
+	switch node.Kind {
+	case yaml.MappingNode:
+		tasks := orderedmap.New[string, *Task]()
+		if err := node.Decode(&tasks); err != nil {
+			return err
+		}
+
+		// nolint: errcheck
+		tasks.Range(func(name string, task *Task) error {
+			// Set the task's name
+			if task == nil {
+				task = &Task{
+					Task: name,
+				}
+			}
+			task.Task = name
+
+			// Set the task's location
+			for _, keys := range node.Content {
+				if keys.Value == name {
+					task.Location = &Location{
+						Line:   keys.Line,
+						Column: keys.Column,
+					}
+				}
+			}
+			tasks.Set(name, task)
+			return nil
+		})
+
+		*t = Tasks{
+			OrderedMap: tasks,
+		}
+		return nil
+	}
+
+	return fmt.Errorf("yaml: line %d: cannot unmarshal %s into tasks", node.Line, node.ShortTag())
+}