[htmx 4] `event` missing from `hx-vals` JS expression

I used to have access to `event` within the `hx-vals` attribute when using the `js:` prefix, so I could pull some data out of a custom event's `detail`. It doesn't seem like that functionality is available any longer in v4. Not sure if that's intended or an oversight (hopefully not the latter!).

Start by reproducing the reported behavior in htmx 4 with an hx-vals js: expression that reads event.detail from a custom event. Compare it with the previous behavior and determine whether event access is intended; done means the expected behavior is documented by a regression test or the issue is clarified as intentional.

Written by the indexing model from the issue text.