allow window stack to shrink its width again

The functionality to equalize the width all windows in a stack only
allows to grow the width so far. This was basically the result of the
mistake to use the window width for comparison. Once the stack adapted
its size the first time, it will always just continue to grow by
definition.
The solution to this is to use the buffer width of eachs window in the
stack. This will then also shrink all windows again.

lua/code_action_menu/windows/stacking_window.lua

@@ -118,13 +118,11 @@ function StackingWindow:after_opened()
   local last_window = self.window_stack[#self.window_stack]
 
   if not last_window.is_anchor then
-    local own_width = vim.api.nvim_win_get_width(self.window_number)
-    local last_window_width = vim.api.nvim_win_get_width(
-      last_window.window_number
-    )
+    local own_width = buffer_utils.get_buffer_width(self.buffer_number)
+    local last_width = buffer_utils.get_buffer_width(last_window.buffer_number)
 
-    if last_window_width >= own_width then
-      self:set_window_width(last_window_width)
+    if last_width >= own_width then
+      self:set_window_width(last_width)
     else
       for _, window in ipairs(self.window_stack) do
         if not window.is_anchor then