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.lua

@@ -936,7 +936,7 @@ local function buf_attach(bufnr)
   local group = api.nvim_create_augroup(augroup, { clear = true })
   api.nvim_create_autocmd('BufWritePre', {
     group = group,
-    buffer = bufnr,
+    buf = bufnr,
     desc = 'vim.lsp: textDocument/willSave',
     callback = function(ctx)
       for _, client in ipairs(lsp.get_clients({ bufnr = ctx.buf })) do
@@ -963,7 +963,7 @@ local function buf_attach(bufnr)
   })
   api.nvim_create_autocmd('BufWritePost', {
     group = group,
-    buffer = bufnr,
+    buf = bufnr,
     desc = 'vim.lsp: textDocument/didSave handler',
     callback = function(ctx)
       text_document_did_save_handler(ctx.buf)