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/eval.h

@@ -63,7 +63,6 @@ enum {
     VV_OLDFILES,
     VV_WINDOWID,
     VV_PROGPATH,
-    VV_JOB_DATA,
     VV_COMMAND_OUTPUT,
     VV_LEN, /* number of v: vars */
 };