Improve pretty logger

[fpawel]

Golals:

• correct processing of structural logging groups
• correct implementation of the interface interface { WithGroup(name string) slog.Handler } in accordance with slog contract
• additional logger options useful for local debugging

for example the following statement

log.WithGroup("group1").WithGroup("group2").Info("msg", slog.Group("group3", slog.Group("group4", "k1", "v1", "k2", "v2")))

produces something like the following output with the original version of the logger from the current main branch

[20:16:16.624] INFO: msg {
  "group3": [
    {
      "Key": "group4",
      "Value": {}
    }
  ]
}

and more meaningful output with corrected logger

INFO msg {"group1":{"group2":{"group3":{"group4":{"k1":"v1","k2":"v2"}}}}} main.go:31.main

Pease don't forget to squash before merge