vim-patch:9.1.0934: hard to view an existing buffer in the preview window (#31605)

Problem:  hard to view an existing buffer in the preview window
Solution: add the :pbuffer command (Yinzuo Jiang)

Similar as `:pedit` and `:buffer` command. `:pbuffer` edits buffer [N]
from the buffer list in the preview window.

`:pbuffer` can also open special buffer, for example terminal buffer.

closes: vim/vim#16222

a2a2fe841e

Cherry-pick Test_popup_and_previewwindow_dump() changes from patch
9.0.0625.
Cherry-pick Run_noroom_for_newwindow_test() changes from patches
8.2.0432 and 9.0.0363.

Co-authored-by: Yinzuo Jiang <jiangyinzuo@foxmail.com>

src/nvim/ex_cmds.lua

@@ -1946,6 +1946,12 @@ M.cmds = {
     addr_type = 'ADDR_NONE',
     func = 'ex_packloadall',
   },
+  {
+    command = 'pbuffer',
+    flags = bit.bor(BANG, RANGE, BUFNAME, BUFUNL, COUNT, EXTRA, CMDARG, TRLBAR),
+    addr_type = 'ADDR_BUFFERS',
+    func = 'ex_pbuffer',
+  },
   {
     command = 'pclose',
     flags = bit.bor(BANG, TRLBAR),