improve nav tree lookups

[davidmarkclements]

the entire navigation tree is queried synchronously on start up (including all node_modules),
then the content of each file is saved into memory

obviously this is an out of memory error waiting to happen, so we need to

1. load all files paths and content from debugger api ONLY
2. all other paths and content to be lazily loaded upon selection (see scope tree implementation in reducers/actions for an example of how to)

[davidmarkclements]

another issue here is remote (or containerized) file systems (since we support the <host> arg)

not only do we need to make this lazy, we also need to extract the fs info through the process we're debugging - (e.g. by sending v8 proto eval of fs ops, getting back <ContextMirror> ,looking those up to get the dir/file contents)