[vim] Open fzf in a tmux or Zellij floating pane by default (#4905)

The window fzf was started from stays visible, and can be used while fzf
runs, which a popup inside Vim cannot offer.

Only where the pane is not modal, so tmux 3.7 or above, or Zellij 0.44 or
above with fzf 0.71.0 or above. A tmux popup from 3.3 to 3.6 cannot be left,
and below 3.3 fzf goes through the fzf-tmux script, whose options are
spelled differently. Both keep the window inside Vim. An explicit popup
layout is unaffected and still works from 3.3.

fzf checks tmux before Zellij, so tmux wins when both are set.

g:fzf_layout still wins, so anyone who set it sees no change.

CHANGELOG.md

@@ -14,6 +14,9 @@ CHANGELOG
       ```vim
       let g:fzf_layout = { 'popup': '90%,70%' }
       ```
+    - fzf now opens in a tmux or Zellij floating pane by default, so the window it was started from stays visible and can be used while fzf is running
+        - Requires tmux 3.7+ or Zellij 0.44+
+        - Set `g:fzf_layout` to pick a different layout
 
 0.74.3
 ------