panes: $SLOSH_PANE, so a program knows which pane it is in

A program in a pane could name every pane but its own. `panes` says which
one is *focused*, and focus belongs to the human, so anything that inferred
"where do I live" from it did its work in whichever tab somebody else
happened to be looking at -- and lost a race with anybody else asking the
same question.

So export the id into the child, the way tmux has $TMUX_PANE and zellij
$ZELLIJ_PANE_ID. The id is taken before the pane is made, because pane_new
spawns the program and assigning it afterwards is one instruction too late;
it lives on the pane rather than the layout, so a rerun keeps it -- an
identity that changed under a rerun would be no identity at all. Unset, not
zero, when there is none: a stale value inherited from an outer session
would read as this one's.

`panes` also reports each pane's pid now, which is the same question
answered for anything that cannot read its own environment: walk your
parents up until one of them is on that list.

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