Implement `as_str` & `Display` for all operator enums (#10691)
## Summary
This PR adds the `as_str` implementation for all the operator methods.
It already exists for `CmpOp` which is being [used in the
linter](https://github.com/astral-sh/ruff/blob/ffcd77860c316bfe5709551389eb52e5feb702f6/crates/ruff_linter/src/rules/flake8_simplify/rules/key_in_dict.rs#L117)
and it makes sense to implement it for the rest as well. This will also
be utilized in error messages for the new parser.
