Set --border-label as the title of the tmux floating pane (#4853)

The pane sets the options itself before running fzf, so that they are
in place no matter how quickly the command exits, targeted at
$TMUX_PANE; the default target would resolve to the active pane of
the session's current window.

- pane-border-format is set to '#{pane_title}' so that the label is
  displayed on the border when pane-border-status is enabled;
  pane-border-status itself is a window option in released tmux
  versions and is left alone
- When a border style is explicitly specified with --border, a popup
  is used instead of a floating pane so that the fzf-drawn border is
  the only border shown; give 'border-native' to force a floating pane
- 'none' and 'line' are treated as no border; fzf draws no box for
  either, so the label is displayed on the native border
- Remove 'border-fzf' which is now redundant; it was never released
- The title is escaped for select-pane -T which expands format
  expressions; a lone ';' is escaped as tmux would treat it as a
  command separator
- The label is skipped when ANSI stripping leaves an empty string
- --border-label-pos is ignored
- Fix remain-on-exit set on the original pane instead of the floating
  pane

src/zellij.go

@@ -11,7 +11,7 @@ func runZellij(args []string, opts *Options) (int, error) {
 		"run", "--floating", "--close-on-exit", "--block-until-exit",
 		"--cwd", dir,
 	}
-	if opts.Tmux.border != tmuxBorderNative {
+	if !opts.Tmux.border {
 		zellijArgs = append(zellijArgs, "--borderless", "true")
 	}
 	switch opts.Tmux.position {