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/vim/_meta/api_keysets_extra.lua

@@ -65,7 +65,7 @@ error('Cannot require a meta file')
 --- @field once? boolean
 --- @field pattern? string
 --- @field buflocal? boolean
---- @field buffer? integer
+--- @field buf? integer
 
 --- @class vim.api.keyset.create_autocmd.callback_args
 --- @field id integer autocommand id
@@ -170,7 +170,7 @@ error('Cannot require a meta file')
 
 --- @class vim.api.keyset.get_keymap
 --- @field abbr? 0|1
---- @field buffer? 0|1
+--- @field buf? 0|1
 --- @field callback? function
 --- @field desc? string
 --- @field expr? 0|1