feat: Check directory structure

[Rotzbua]

The recommended directory structure is:
https://nodered.org/docs/creating-nodes/packaging#directory-structure

Some projects do not follow this structure and put every node into the root directory.

[knolleary]

@Rotzbua the docs describe the 'typical' directory structure. They also say:

There are no strict requirements over the directory structure used within the package.

So I believe it would be inappropriate to penalise a module for this.

[Rotzbua]

Well the term "no strict requirements" means just, that the validator has not to fail if the directory structure does not match. There could be still a warning or information about the typical/recommended directory.

[knolleary]

There are many different ways the node files could be organised. We should only warn if there is something that we consider 'bad' - but I'm not sure what that would be.