feat(api): nvim_open_win() relative to tabline and laststatus #32006

Problem:  Anchoring a floating window to the tabline and laststatus is
          cumbersome; requiring autocommands and looping over all
          windows/tabpages.
Solution: Add new "tabline" and "laststatus" options to the `relative`
          field of nvim_open_win() to place a window relative to.

src/nvim/buffer_defs.h

@@ -900,6 +900,8 @@ typedef enum {
   kFloatRelativeWindow = 1,
   kFloatRelativeCursor = 2,
   kFloatRelativeMouse = 3,
+  kFloatRelativeTabline = 4,
+  kFloatRelativeLaststatus = 5,
 } FloatRelative;
 
 /// Keep in sync with win_split_str[] in nvim_win_get_config() (api/win_config.c)