[Enh]: add `Expr.first`

I often find myself desiring a .first() expression. Is there a plan to support that expression?

Polars Example
```py
df = pl.DataFrame(
    {
        "group": ["A", "A", "A", "B", "B", "B"],
        "value": [1, 2, 3, 4, 5, 6],
    }
)
df = df.with_columns(
    pl.col("value").first().over("group").alias("mean_value")
)
```

## Related
- [ ] #1793
- [ ] #2534