backport: feat(api): rename buffer to buf (#38899)

feat(api): rename buffer to buf

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: Jordan <46637683+JordanllHarper@users.noreply.github.com>

runtime/lua/editorconfig.lua

@@ -165,7 +165,7 @@ function properties.trim_trailing_whitespace(bufnr, val)
   if val == 'true' then
     vim.api.nvim_create_autocmd('BufWritePre', {
       group = 'nvim.editorconfig',
-      buffer = bufnr,
+      buf = bufnr,
       callback = function()
         local view = vim.fn.winsaveview()
         vim.api.nvim_command('silent! undojoin')
@@ -177,7 +177,7 @@ function properties.trim_trailing_whitespace(bufnr, val)
     vim.api.nvim_clear_autocmds({
       event = 'BufWritePre',
       group = 'nvim.editorconfig',
-      buffer = bufnr,
+      buf = bufnr,
     })
   end
 end
@@ -194,7 +194,7 @@ function properties.insert_final_newline(bufnr, val)
   if vim.bo[bufnr].endofline ~= endofline then
     vim.api.nvim_create_autocmd('BufWritePre', {
       group = 'nvim.editorconfig',
-      buffer = bufnr,
+      buf = bufnr,
       once = true,
       callback = function()
         vim.bo[bufnr].endofline = endofline