Feature request: Solution.to_phreeqc()

Per discussion today, let's add a `Solution.to_phreeqc()` method that creates a PHREEQC-compatible input file. Similar to [`to_file()`](https://github.com/KingsburyLab/pyEQL/blob/56b71d62bcf9cec3a9ae586470414ec801ad281c/src/pyEQL/solution.py#L2479), the method should take one argument which is the filename to write out to.