Summary

Adds the empty struct idiom to the idioms section.

The empty struct (struct{}) is a well-known Go idiom widely used in the community, but it hasn't been documented in this repository yet.
This PR adds idiom/empty-struct.md with some common use cases.

Changes

• add idiom/empty-struct.md with implementation and usage examples
• update README.md to include the new pattern in the Idioms table