core: Add %D specifier for $XDG_DATA_HOME

core: Add %D specifier for $XDG_DATA_HOME

We already have specifiers that resolve to $XDG_STATE_HOME, and
$XDG_CONFIG_HOME. $XDG_DATA_HOME is in a similar vein.

It allows units belonging to the user service manager to correctly look
into ~/.local/share. I imagine this would be most useful inside of
condition checks (i.e. only run a service on session startup if some
data is not found in ~/.local/share) or in the inotify monitoring of a
.path unit