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/lsp/completion.lua

@@ -774,7 +774,7 @@ end
 local function on_completechanged(group, bufnr)
   api.nvim_create_autocmd('CompleteChanged', {
     group = group,
-    buffer = bufnr,
+    buf = bufnr,
     callback = function(ev)
       local completed_item = vim.v.event.completed_item or {}
       local lsp_item = vim.tbl_get(completed_item, 'user_data', 'nvim', 'lsp', 'completion_item')
@@ -920,13 +920,13 @@ end
 ---@return integer
 local function register_completedone(bufnr)
   local group = api.nvim_create_augroup(get_augroup(bufnr), { clear = false })
-  if #api.nvim_get_autocmds({ buffer = bufnr, event = 'CompleteDone', group = group }) > 0 then
+  if #api.nvim_get_autocmds({ buf = bufnr, event = 'CompleteDone', group = group }) > 0 then
     return group
   end
 
   api.nvim_create_autocmd('CompleteDone', {
     group = group,
-    buffer = bufnr,
+    buf = bufnr,
     callback = function()
       local reason = api.nvim_get_vvar('event').reason ---@type string
       if reason == 'accept' then
@@ -1033,9 +1033,7 @@ local function trigger(bufnr, clients, ctx)
     Context.cursor = { cursor_row, start_col }
     if #matches > 0 and has_completeopt('popup') then
       local group = get_augroup(bufnr)
-      if
-        #api.nvim_get_autocmds({ buffer = bufnr, event = 'CompleteChanged', group = group }) == 0
-      then
+      if #api.nvim_get_autocmds({ buf = bufnr, event = 'CompleteChanged', group = group }) == 0 then
         on_completechanged(group, bufnr)
       end
     end
@@ -1148,7 +1146,7 @@ local function enable_completions(client_id, bufnr, opts)
     local group = register_completedone(bufnr)
     api.nvim_create_autocmd('LspDetach', {
       group = group,
-      buffer = bufnr,
+      buf = bufnr,
       desc = 'vim.lsp.completion: clean up client on detach',
       callback = function(ev)
         disable_completions(ev.data.client_id, ev.buf)
@@ -1158,14 +1156,14 @@ local function enable_completions(client_id, bufnr, opts)
     if opts.autotrigger then
       api.nvim_create_autocmd('InsertCharPre', {
         group = group,
-        buffer = bufnr,
+        buf = bufnr,
         callback = function()
           on_insert_char_pre(buf_handles[bufnr])
         end,
       })
       api.nvim_create_autocmd('InsertLeave', {
         group = group,
-        buffer = bufnr,
+        buf = bufnr,
         callback = on_insert_leave,
       })
     end