feat: add merge function

Able to deal with:
1.  merge a subtree

packages/file-graph/fileNode.js

@@ -11,11 +11,15 @@ export default class FileNode extends File {
   addChild(fileInput, { makeParents } = {}) {
     const file = new FileNode(fileInput)
     if (file.path === this.path) {
-      throw new DuplicateItemError('File already exists in the tree')
+      throw new DuplicateItemError(
+        `File ${file.path} already exists in the tree: ${this.toString()}`
+      )
     }
 
     if (this.children.some((child) => child.path === file.path)) {
-      throw new DuplicateItemError('File already exists in the tree')
+      throw new DuplicateItemError(
+        `File ${file.path} already exists in the tree: ${this.toString()}`
+      )
     }
 
     const parentPath = dirname(file.path)
@@ -37,11 +41,47 @@ export default class FileNode extends File {
         })
         this.addChild(parent, { makeParents })
       } else {
-        throw new ParentNotExistsError('Parent path not found in the tree')
+        throw new ParentNotExistsError(
+          `Parent path not found in the tree, ${file.path}, ${this.path}`
+        )
       }
     }
   }
 
+  merge(fileNode) {
+    // Merge the children of the provided file node into this node
+    // If a child already exists, merge the children of that child
+    // If a child does not exist, add it to this node
+    if (fileNode.path === this.path) {
+      fileNode.children.forEach((child) => {
+        const existingChild = this.children.find((c) => c.path === child.path)
+        if (existingChild) {
+          existingChild.merge(child)
+        } else {
+          this.children.push(child)
+        }
+      })
+
+      // If a child exists in this node but not in the provided node, remove it
+      this.children = this.children.filter((child) =>
+        fileNode.children.some((c) => c.path === child.path)
+      )
+    } else if (this.contains(fileNode)) {
+      const childNode = this.children.find((child) => child.contains(fileNode))
+      if (childNode) {
+        childNode.merge(fileNode)
+      } else {
+        this.addChild(fileNode)
+      }
+    } else {
+      throw new Error(
+        'Cannot merge file nodes with different paths',
+        this.path,
+        fileNode.path
+      )
+    }
+  }
+
   toString() {
     const indent = '    '.repeat(this.path.split('/').length - 1)
     const suffix = this.isDirectory() ? '/' : ''