unnecessary-literal-set (C405): also warn about `set({})`

### Summary

I have seen this a few times. It would be nice if [C405](https://docs.astral.sh/ruff/rules/unnecessary-literal-set/) would warn about this case as well. `{...}` is a literal set so seems reasonable to warn about `{}` in addition to `[]`.

Start with the linked C405 rule documentation and locate the rule's implementation and tests. Compare the existing handling of the [] case with the requested {} case; done means C405 also warns about set({}) consistently with literal-set handling.

Written by the indexing model from the issue text.