feat(api): rename buffer to buf #35330

Problem:
`:help dev-name-common` states that "buf" should be used instead of
"buffer" but there are cases where buffer is mentioned in the lua API.

Solution:
- Rename occurrences of "buffer" to "buf" for consistency with the
  documentation.
- Support (but deprecate) "buffer" for backwards compatibility.

Co-authored-by: Justin M. Keyes <justinkz@gmail.com>

runtime/lua/vim/pack.lua

@@ -1155,7 +1155,7 @@ local function show_confirm_buf(lines, on_finish)
     delete_buffer()
   end
   -- - Use `nested` to allow other events (useful for statuslines)
-  api.nvim_create_autocmd('BufWriteCmd', { buffer = bufnr, nested = true, callback = finish })
+  api.nvim_create_autocmd('BufWriteCmd', { buf = bufnr, nested = true, callback = finish })
 
   -- Define action to cancel confirm
   --- @type integer