API: dt64 resolution for PeriodIndex.to_timestamp

```python
idx = PeriodIndex(["2011-01", "NaT", "2011-02"], freq="2M", name="idx")

res = idx.to_timestamp()
```


ATM we always give datetime64[ns] resolution in to_timestamp, but we could reasonably choose lower-resolutions for lower-resolution PeriodDtypes.  This could avoid OutOfBoundsDatetime in some cases for e.g. `pd.PeriodIndex(["May 3000"], freq="M")`
