feat!: Write logs to $XDG_STATE_HOME

[ianlewis]

This PR updates the path that the hook.log gets written by default from $XDG_CONFIG_HOME/sbp to $XDG_STATE_HOME/sbp/log.

This is more in line with the XDG Base Directory Specification:

The $XDG_STATE_HOME contains state data that should persist between (application) restarts, but that is not important or portable enough to the user that it should be stored in $XDG_DATA_HOME. It may contain:

• actions history (logs, history, recently used files, …)

$XDG_STATE_HOME/sbp/log was chosen because $XDG_STATE_HOME/sbp could contain different types of state in the future and I think it makes sense to give logs their own directory.

Fixes #141

[brujoand]

A nice catch 👍