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/taskfile.go

@@ -0,0 +1,80 @@
+package ast
+
+import (
+	"fmt"
+	"time"
+
+	"github.com/Masterminds/semver/v3"
+	"gopkg.in/yaml.v3"
+
+	"github.com/go-task/task/v3/errors"
+)
+
+var V3 = semver.MustParse("3")
+
+// Taskfile is the abstract syntax tree for a Taskfile
+type Taskfile struct {
+	Location string
+	Version  *semver.Version
+	Output   Output
+	Method   string
+	Includes *IncludedTaskfiles
+	Set      []string
+	Shopt    []string
+	Vars     *Vars
+	Env      *Vars
+	Tasks    Tasks
+	Silent   bool
+	Dotenv   []string
+	Run      string
+	Interval time.Duration
+}
+
+func (tf *Taskfile) UnmarshalYAML(node *yaml.Node) error {
+	switch node.Kind {
+	case yaml.MappingNode:
+		var taskfile struct {
+			Version  *semver.Version
+			Output   Output
+			Method   string
+			Includes *IncludedTaskfiles
+			Set      []string
+			Shopt    []string
+			Vars     *Vars
+			Env      *Vars
+			Tasks    Tasks
+			Silent   bool
+			Dotenv   []string
+			Run      string
+			Interval time.Duration
+		}
+		if err := node.Decode(&taskfile); err != nil {
+			return err
+		}
+		tf.Version = taskfile.Version
+		tf.Output = taskfile.Output
+		tf.Method = taskfile.Method
+		tf.Includes = taskfile.Includes
+		tf.Set = taskfile.Set
+		tf.Shopt = taskfile.Shopt
+		tf.Vars = taskfile.Vars
+		tf.Env = taskfile.Env
+		tf.Tasks = taskfile.Tasks
+		tf.Silent = taskfile.Silent
+		tf.Dotenv = taskfile.Dotenv
+		tf.Run = taskfile.Run
+		tf.Interval = taskfile.Interval
+		if tf.Version == nil {
+			return errors.New("task: 'version' is required")
+		}
+		if tf.Vars == nil {
+			tf.Vars = &Vars{}
+		}
+		if tf.Env == nil {
+			tf.Env = &Vars{}
+		}
+		return nil
+	}
+
+	return fmt.Errorf("yaml: line %d: cannot unmarshal %s into taskfile", node.Line, node.ShortTag())
+}