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.

Problem:
Tests that need to check `nvim` CLI behavior (no RPC session) create
their own ad-hoc `system()` wrappers.

Solution:
- Use `n.spawn_wait` instead of `system()`.
- Bonus: this also improves the tests by explicitly checking for
  `stdout` or `stderr`. And if a signal is raised, `ProcStream.status`
  will reflect it.

Problem:
Can't use `n.clear()` to test non-RPC `nvim` invocations. So tests end
up creating ad-hoc wrappers around `system()` or `jobstart()`.

Solution:
- Introduce `n.spawn_wait()`
- TODO (followup PR): Rename `n.spawn()` and `n.spawn_wait()`.
  It's misleading that `n.spawn()` returns a RPC session...

Problem:
`termopen` has long been a superficial wrapper around `jobstart`, and
has no real purpose. Also, `vim.system` and `nvim_open_term` presumably
will replace all features of `jobstart` and `termopen`, so centralizing
the logic will help with that.

Solution:
- Introduce `eval/deprecated.c`, where all deprecated eval funcs will live.
- Introduce "term" flag of `jobstart`.
- Deprecate `termopen`.

When a terminal application running inside the terminal emulator sets
the cursor shape or blink status of the cursor, update the cursor in the
parent terminal to match.

This removes the "virtual cursor" that has been in use by the terminal
emulator since the beginning. The original rationale for using the
virtual cursor was to avoid having to support additional UI methods to
change the cursor color for other (non-TUI) UIs, instead relying on the
TermCursor and TermCursorNC highlight groups.

The TermCursor highlight group is now used in the default 'guicursor'
value, which has a new entry for Terminal mode. However, the
TermCursorNC highlight group is no longer supported: since terminal
windows now use the real cursor, when the window is not focused there is
no cursor displayed in the window at all, so there is nothing to
highlight. Users can still use the StatusLineTermNC highlight group to
differentiate non-focused terminal windows.

BREAKING CHANGE: The TermCursorNC highlight group is no longer supported.

In release builds, the Compilation: line is omitted so the build is reproducible. Since the "fall-back for $VIM" line is always present, check for that instead.

Co-authored-by: zeertzjq <zeertzjq@outlook.com>