Simplify vim pane id extraction

Previously, would do this:
1. from shell pane, spawn vim pane and capture pane index
2. from shell pane, list panes, and extract pane id with regexes

But once the vim pane is created, the pane id is available in $TMUX_PANE, so all
we need to do is, before we actually execute vim, have the vim pane call:

    tmux set-environment $window_key $TMUX_PANE

Much simpler this way.