Run fzf asynchronously in the Vim plugin

Popup mode held the fzf process with system(), which froze Vim until fzf
exited. fzf in a popup draws in a pane of its own, so that process only
waits for it and does not need a window. Hold it with a job instead and
Vim keeps processing its event loop, which is what a live preview needs.
Nothing is displayed for the job. Falls back to the blocking path when
the job cannot start, so the sink still runs and temp files are removed.

- job_start() sets $TERM=dumb and the popup inherits the environment, so
  fzf dropped to its 16-color scheme. Restore it via 'env', or in the
  command itself before 8.0.902, when 'env' was added
- Fullscreen now uses a terminal buffer in a new tab on Vim too. use_term
  lacked parentheses, so && bound tighter than || and the layout test was
  dead on Neovim, which already behaved this way
- fzf#run returns an empty list in these modes. Callers use sink,
  sinklist or exit, and the vader specs now wait for completion
- Append --no-tmux only when the spec asks for a Vim window, so --popup
  in $FZF_DEFAULT_OPTS survives a spec with no layout option

Accept popup as a synonym of the tmux layout key, matching --popup being
the name of --tmux. popup wins when both are given.

s:tmux_enabled():

- Accept $ZELLIJ, which --popup covers as well
- Parse tmux -V with matchstr and compare with s:compare_versions. The
  old string comparison against 'tmux 1.7' misreads 10.0
- Drop the fzf-tmux requirement on tmux 3.3 or above, where --tmux needs
  no script. Removing the script silently disabled popups entirely
- Resolve the script where it is used, and anchor the legacy test to ^-
  so a --tmux value containing a dash, as in 90%,60%,border-native, is
  not mistaken for a legacy flag

doc/fzf.txt

@@ -310,7 +310,7 @@ The following table summarizes the available options.
   `options`                   | string/list   | Options to fzf
   `dir`                       | string        | Working directory
   `up` / `down` / `left` / `right`  | number/string | (Layout) Window position and size (e.g.  `20` ,  `50%` )
-  `tmux`                      | string        | (Layout) `--tmux` options (e.g.  `90%,70%` )
+  `popup` / `tmux`              | string        | (Layout) `--popup` options (e.g.  `90%,70%` )
   `window`  (Vim 8 / Neovim)  | string        | (Layout) Command to open fzf window (e.g.  `vertical aboveleft 30new` )
   `window`  (Vim 8 / Neovim)  | dict          | (Layout) Popup window settings (e.g.  `{'width': 0.9, 'height': 0.6}` )
  ---------------------------+---------------+----------------------------------------------------------------------
@@ -467,13 +467,14 @@ in Neovim.
     "   - 'rounded' / 'sharp' / 'horizontal' / 'vertical' / 'top' / 'bottom' / 'left' / 'right'
     let g:fzf_layout = { 'window': { 'width': 0.9, 'height': 0.6 } }
 <
-Alternatively, you can make fzf open in a tmux popup window (requires tmux 3.2
-or above) by putting `--tmux` options in `tmux` key.
+Alternatively, you can make fzf open in a popup window (requires tmux 3.3 or
+above, or Zellij 0.44 or above) by putting `--popup` options in `popup` key.
+`tmux` is accepted as a synonym, just as `--tmux` is an alias of `--popup`.
 >
-    " See `--tmux` option in `man fzf` for available options
+    " See `--popup` option in `man fzf` for available options
     " [center|top|bottom|left|right][,SIZE[%]][,SIZE[%]]
-    if exists('$TMUX')
-      let g:fzf_layout = { 'tmux': '90%,70%' }
+    if exists('$TMUX') || exists('$ZELLIJ')
+      let g:fzf_layout = { 'popup': '90%,70%' }
     else
       let g:fzf_layout = { 'window': { 'width': 0.9, 'height': 0.6 } }
     endif