feat(api): add 'buffer' argument to nvim_get_autocmds (#17594)

This enables retrieving autocommands defined in the given buffers. Under
the hood this simply translates the buffer numbers into '<buffer=%d>'
patterns.

src/nvim/api/keysets.lua

@@ -142,6 +142,7 @@ return {
     "event";
     "group";
     "pattern";
+    "buffer";
   };
   create_augroup = {
     "clear";