eval: Refactor vimscript job control API

- Remove JobActivity autocmd and v:job_data variable
- Simplify `jobstart` to receive:
  - An argument vector
  - An optional dictionary which may contain any of the current `jobstart`
    options plus `on_stdout`, `on_stderr` and `on_exit` callbacks.
- Refactor and add more job tests
- Update documentation

src/nvim/msgpack_rpc/channel.c

@@ -347,7 +347,7 @@ static void job_err(RStream *rstream, void *data, bool eof)
   }
 }
 
-static void job_exit(Job *job, void *data)
+static void job_exit(Job *job, int status, void *data)
 {
   decref(data);
 }