Multi Instance: Refactor: Node classes

[alex-courtis]

• Multi Instance - Contributors Wanted - Please Take An Issue #2255

Node classes are defined in node.lua however are created as tables.

Add Node classes and constructors.

This may not be necessary for multi instance.

nvim-tree.explorer.node is not explorer specific and should go into the Node class. It might be advisable to create a PR for just those class members first.

[alex-courtis]

Classes created #2916

TODO:

• Split Link into DirectoryLink and FileLink
• Move methods from BaseNode into children
• change link_to to absolute_path_target
• Tighten members e.g. parent must be a DirectoryNode
• Remove the Node alias, renaming BaseNode to Node
• Remove tests such as #node.nodes to determine directory