[refactoring] remove general Node struct

pkg/ast/ast.go

@@ -1,8 +1,10 @@
 package ast
 
+import "github.com/z7zmey/php-parser/pkg/position"
+
 type Vertex interface {
 	Accept(v NodeVisitor)
-	GetNode() *Node
+	GetPosition() *position.Position
 }
 
 type Traverser interface {

pkg/ast/visitor/dump_test.go

@@ -20,12 +20,6 @@ func ExampleDump() {
 					},
 				},
 			},
-			Position: &position.Position{
-				StartLine: 1,
-				EndLine:   1,
-				StartPos:  0,
-				EndPos:    1,
-			},
 		},
 		Stmts: []ast.Vertex{
 			&ast.Identifier{},