Support any valid target-pane string in VimuxRunnerIndex

[btolsch]

This change makes s:hasRunner() effectively query tmux directly for whether the runner exists. This works for:

• Any form of target-pane as described in man tmux, e.g.:
  • Local pane index e.g. 0, 1, 2, etc.
  • Pane ID e.g. %1, %28, etc.
• Any form of target-window as described in man tmux, when local pane 0 will be used, e.g.:
  • Window ID e.g. @1, @7
  • window_index.pane_index e.g. :3.2

The implementation calls tmux send-keys -t <target_pane> with no keys, so tmux's exit code determines whether the pane is found but no keys are actually sent to the pane.

I tend to use a local pane index or sometimes a window.pane index, which was broken recently when I updated (I think by 383d711).

[mvanderkamp]

Excellent! I tested this out, and I like it a lot! Thank you for the contribution!