slog default logger: replace and then revert

Before `slog.SetDefault`, it's sort of the case that the `log` package's default resource is the destination for all global/default/standard logging (from `slog` or `log`). The mechanism is a little aggressive, even, in the details - there is a slog handler that outputs via a resource managed by the log function. The function is made visible via a pointer held in a `log/internal` package log and slog both import.

After `slog.SetDefault`, the opposite configuration occurs in the sense that `log` sends its output into `slog`. I don't believe this flip is revertible.

If you must have `log` formatting, I think the thing that might be possible to implement a `slog` handler with formatting that precisely matched the `log`/pre-`SetDefault` arrangement. Dunno if that implementation exists already somewhere. An easier bar would be to set a `slog.NewTextHandler` as default. Hope this helps.