Make Synset.ili an optional string; add wn.ili module

[goodmami]

Is your feature request related to a problem? Please describe.

The Synset.ili attribute returns an ILI object. Before, this caused problems when trying to use it directly in a call, like wn.synsets(ili=ss.ili). Recent versions allow this to work as well, but it might still be good to simplify things.

Describe the solution you'd like

Instead of returning an ILI object, Synset.ili would just return a string or None. In order to get ILI status, definitions, or metadata (which is really metadata of the ILI definition), we could have an wn.ili module with relevant methods. I suspect that people rarely get these kinds of data, so it's probably fine to tuck it in a module.

Additional context

This would be a breaking change, so best to only do it once for the v1.0 release.