feat(api): enable nvim_exec_autocmds to pass arbitrary data (#18613)

Add a "data" key to nvim_exec_autocmds that passes arbitrary data (API
objects) to autocommand callbacks.

src/nvim/api/keysets.lua

@@ -146,6 +146,7 @@ return {
     "group";
     "modeline";
     "pattern";
+    "data";
   };
   get_autocmds = {
     "event";