Consistent behavior when XDG_DATA_HOME is specified

[VojtechStep]

Currently, when the environment variable XDG_DATA_HOME is not
specified, autojump-rs puts its data files under
~/.local/share/autojump/..., but if it is specified, it puts it into
the directory directly - no autojump folder is created.

I feel this behavior is inconsistent, because it does different things
when the data home is set to its default value, ~/.local/share.

I think we should always use a separate directory for autojump inside
whatever data directory is specified.

[VojtechStep]

Also, after looking at the source code of the original autojump, they
always create their directory, so this is a needless deviation from
the tool you're trying to replace and it's not documented, so I think
this should be considered a legitimate bug.