Add autocomplete for operator

To avoid mistakes it would be great to have operator autocomplete in types.

We could either just use a union type and then allow also for any string to account for custom operators, or we could be strict and pair each operator with its corresponding value (e.g. `lessThan` needs a `value: number`, and we could document them so users know how to extend that with their custom types.

If there is interest in this I'd be happy to contribute the PR.

Thanks in advance.

Start with the issue proposal and inspect the repository's existing type definitions and operator handling. Compare the proposed union-based autocomplete with paired operator/value types, including how custom operators should be extended. Done means the chosen design provides operator autocomplete and documents custom types clearly.

Written by the indexing model from the issue text.