ENH: Implement sem for Rolling and Expanding
- [x] closes #26476
- [x] tests added / passed
- [x] passes `black pandas`
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [x] whatsnew entry

I added sem to the Rolling and Expanding API as mentioned in the issue linked above.
Thanks. Could you also add tests for `groupby().rolling().sem()` and `groupby().expanding().sem()`? Done, count also counts for the grouping column. Is this the expected behavior? Could you give a small example for the count behavior? The snippet

```
df = pd.DataFrame([["a", 1], ["a", 2], ["b", 1], ["b", 2], ["b", 3]], columns=["a", "b"])
result = df.groupby("a").rolling(2, min_periods=1).count()
print(result)
```

returns 

```
       a    b
a            
a 0  1.0  1.0
  1  2.0  2.0
b 2  1.0  1.0
  3  2.0  2.0
  4  2.0  2.0
```

I would not have expected ``a`` to be a column and part of the index. Thanks, I think that's a known bug https://github.com/pandas-dev/pandas/issues/32262 thanks @phofl very nice