Implement splitLevel of Tree.Edit (#705)

When a user positions the cursor within a paragraph and hits the enter
key, the paragraph is divided into two separate paragraphs:

- Original: `<p>a|b</p>`
- Result: `<p>a</p><p>b</p>`

In this commit, `splitLevel` is introduced to support this scenario.

```ts
doc.update((root) => {
  root.tree.edit([0,0], {type:'p',children:[{type:'text', value: 'ab'}]});
  root.tree.edit([2,2], undefined, 1); // split the paragraph
});
```

This commit does not cover all test cases that may arise due to the
split operation. This is because we need to verify the interface
first. we need to add test cases in the simultaneous editing situation
to verify the algorithm later.

api/converter/converter_test.go

@@ -117,7 +117,7 @@ func TestConverter(t *testing.T) {
 						Type:  "text",
 						Value: "Hello world",
 					}},
-				})
+				}, 0)
 
 			return nil
 		})

api/converter/from_pb.go

@@ -519,8 +519,9 @@ func fromTreeEdit(pbTreeEdit *api.Operation_TreeEdit) (*operations.TreeEdit, err
 		parentCreatedAt,
 		from,
 		to,
-		createdAtMapByActor,
 		nodes,
+		int(pbTreeEdit.SplitLevel),
+		createdAtMapByActor,
 		executedAt,
 	), nil
 }

api/converter/to_bytes.go

@@ -271,7 +271,7 @@ func ToTreeNodes(treeNode *crdt.TreeNode) []*api.TreeNode {
 		return pbTreeNodes
 	}
 
-	index.TraverseNode(treeNode.IndexTreeNode, func(node *index.Node[*crdt.TreeNode], depth int) {
+	index.TraverseNode(treeNode.Index, func(node *index.Node[*crdt.TreeNode], depth int) {
 		pbTreeNodes = append(pbTreeNodes, toTreeNode(node.Value, depth))
 	})
 	return pbTreeNodes

api/converter/to_pb.go

@@ -387,8 +387,9 @@ func toTreeEdit(e *operations.TreeEdit) (*api.Operation_TreeEdit_, error) {
 			ParentCreatedAt:     ToTimeTicket(e.ParentCreatedAt()),
 			From:                toTreePos(e.FromPos()),
 			To:                  toTreePos(e.ToPos()),
-			CreatedAtMapByActor: toCreatedAtMapByActor(e.CreatedAtMapByActor()),
 			Contents:            ToTreeNodesWhenEdit(e.Contents()),
+			SplitLevel:          int32(e.SplitLevel()),
+			CreatedAtMapByActor: toCreatedAtMapByActor(e.CreatedAtMapByActor()),
 			ExecutedAt:          ToTimeTicket(e.ExecutedAt()),
 		},
 	}, nil

api/yorkie/v1/resources.proto

@@ -124,6 +124,7 @@ message Operation {
     TreePos to = 3;
     map<string, TimeTicket> created_at_map_by_actor = 4;
     repeated TreeNodes contents = 5;
+    int32 split_level = 7;
     TimeTicket executed_at = 6;
   }
   message TreeStyle {