function getFile(path, tree) {
	const { children } = tree;
	let { url } = tree;
	if (url === path) return tree;
	if (!children) return null;
	const len = children.length;
	for (let i = 0; i < len; i++) {
		const item = children[i];
		const result = getFile(path, item);
		if (result) return result;
	}
	return null;
}
Get all files in a folder
e.g /dir1/dir2/dir3
This function will first test if dir1 exists in the tree,
if not, it will return null, otherwise it will traverse the tree
and return the files in dir3
@param {string} path - Folder path
@param {Tree} tree - Files tree