Problem:
First rtp directory is unpredictable and not in line with XDG
base spec.

Solution:
Use stdpath('data')/spell as directory if 'spellfile' is not set.

Co-authored-by: zeertzjq <zeertzjq@outlook.com>
Co-authored-by: Justin M. Keyes <justinkz@gmail.com>

Always allow the following four events to be nested, as they may contain
important information, and are triggered on the event loop, which may be
processed by a blocking call inside another autocommand.

- ChanInfo
- ChanOpen
- TermRequest
- TermResponse

There are some other events that are triggered on the event loop, but
they are mostly triggered by user actions in a UI client, and therefore
not very likely to happen during another autocommand, so leave them
unchanged for now.

Problem:    Sourcing buffer lines is too complicated.
Solution:   Simplify the code. Make it possible to source Vim9 script lines.
            (Yegappan Lakshmanan, closes vim/vim#9974)

https://github.com/vim/vim/commit/85b43c6cb7d56919e245622f4e42db6d8bee4194

This commit changes the behavior of sourcing buffer lines to always have
a script ID, although sourcing the same buffer always produces the same
script ID.

vim-patch:9.1.0372: Calling CLEAR_FIELD() on the same struct twice

Problem:  Calling CLEAR_FIELD() on the same struct twice.
Solution: Remove the second CLEAR_FIELD().  Move the assignment of
          cookie.sourceing_lnum (zeertzjq).

closes: vim/vim#14627

https://github.com/vim/vim/commit/f68517c1671dfedcc1555da50bc0b3de6d2842f6

Co-authored-by: Yegappan Lakshmanan <yegappan@yahoo.com>

Also change job tests to use `nvim` instead of random programs like `ping`.

Problem:
Cannot detach the current UI.

Solution:
- Introduce `:detach`.
- Introduce `Channel.detach`.

Co-authored-by: bfredl <bjorn.linse@gmail.com>

Related #27764

Problem:
`nvim -es` (and `nvim -Es`) is the recommended way to non-interactively
run commands/vimscript. But it enables shada by default, which is
usually not wanted.

Solution:
- Disable shada by default for `nvim -es/-Es`. This can be overridden by
  `-i foo` if needed.
- Do NOT change the 'loadplugins' default.
  - User config + packages _should_ be enabled by default, for both `nvim
    -es` and `nvim -l`. Else any Lua packages you have can't be accessed
    without `-u path/to/config`, which is clumsy.
      - Use-cases:
        ```
        nvim --headless "+Lazy! sync" +qa
            would become: nvim -es "+Lazy! sync"
        nvim --headless +PlugInstall +qall
            would become: nvim -es +PlugInstall
        ```
  - Opt-out (`--clean` or `-u NONE`) is much easier than opt-in (`-u
    path/to/config`).
  -  User config/packages are analogous to pip packages, which are
     expected when doing `python -c ...`.

related: 7c94bcd2d77e2e54b8836ab8325460a367b79eae
related: ddd0eb6f5120a09b97867d2561ea61309038ccd2

Problem:
- `n.spawn()` is misleading because it also connects RPC, it's not just
  "spawning" a process.
- It's confusing that `n.spawn()` and `n.spawn_argv()` are separate.

Solution:
- Replace `n.spawn()`/`n.spawn_argv()` with a single function `n.new_session()`.
  This name aligns with the existing functions `n.set_session`/`n.get_session`.
    - Note: removes direct handling of `prepend_argv`, but I doubt that was
      important or intentional. If callers want to control use of `prepend_argv`
      then we should add a new flag to `test.session.Opts`.
- Move `keep` to first parameter of `n.new_session()`.
- Add a `merge` flag to `test.session.Opts`
- Mark `_new_argv()` as private. Test should use clear/new_session/spawn_wait
  instead.