Add `.by` parameter to slides, etc., in order to mirror `dplyr` 1.1.0

[brookslogan]

Interacts with #64. Also make sure not to describe epix_slide in #64 as becoming more summarize-like if it's actually going to be more reframe-like (although perhaps the envisioned changes now mean it will be neither, as reframe always ungroups).

[brookslogan]

It looks like .by might use tidyselect rather than data masking; we should make sure to match.

[brookslogan]

Key compatibility question: how does .by work on already-grouped objects?