Support `ls` for inner nodes with children

[j4r0u53k]

Let's say we have node A which also handles its child nodes prop_a_1, prop_a_2 and we want to mount another nodeB into subdir:

* A
  | - prop_a_1
  | - prop_a_2
  \ - subdir
           \ * B
               | - prop_b_1
               \ - prop_b_2

We need that ls called on path A returns all its children: ["prop_a_1", "prop_a_2", "subdir"]

[j4r0u53k]

Consider whether it should be allowed to mount another subtree on a path of a node that provides its own implementation of ls. Such node could alias a static mount by its own subpath.

[j4r0u53k]

Static nodes will not support their own child nodes. Either use separate static mounts for each path or a dynamic mount for a subtree.