add Spring Modulith layer in structural view

[martinlippert]

If a project is a Spring Modulith project, a nested grouping underneath the project one could be established by inspecting the module model and grouping the list of elements by which folder they’re located in.

(cc @odrotbohm)

[martinlippert]

Sample structural view of restbucks sample app:
https://gist.github.com/odrotbohm/9077c8f5a10ea4806e3471ee23a8608e