Add optional logging for X generation

### Motivation

Sometimes an X appears in simulation in an unexpected way, and it can be difficult to trace to the root cause if there's no clear X coming into the logic. For example, a combinational cycle, inferred latch, or contention could cause X generation without any original X or Z obviously causing the X. It would be nice to add a way to identify when that's happening when there's a tricky debug underway.

### Desired solution

Add an *optional* setting in in the ROHD simulator that controls whether or not to log X generation. Note, this has to be only for new X generation and *not* for X propagation, or else the logging would be excessive. Utilizing the existing ROHD logger is probably appropriate.

### Alternatives considered

_No response_

### Additional details

_No response_