Problem:
Event handler declared with `once=true` can re-trigger itself (i.e. more
than once!) by calling `nvim_exec_autocmds` or `:doautocmd`.

Analysis:
This happens because the callback is executed before deletion/cleanup
(`aucmd_del`). And calling `aucmd_del` before `call_autocmd_callback`
breaks the autocmd execution...

Solution:
Set `ac->pat=NULL` to temporarily "delete" the autocmd, then restore it
after executing the callback.

Fix #25526

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

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.

Problem:
- API functions using `try_start` directly, do not surface the
  underlying error message, and instead show generic messages.
- Error-handling code is duplicated in the API impl.
- Failure modes are not tested.

Solution:
- Use `TRY_WRAP`.
- Add tests.

Problem:  The E937 error appears for too short in TextChangedT test.
Solution: Only feed an Enter key after seeing the error.

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

Before calling "attach" a screen object is just a dummy container for
(row, col) values whose purpose is to be sent as part of the "attach"
function call anyway.

Just create the screen in an attached state directly. Keep the complete
(row, col, options) config together. It is still completely valid to
later detach and re-attach as needed, including to another session.

This module is generally used by any tests that need the full Nvim TUI
instead of `screen.lua`. Thus it should live in `functional/` instead of
in `functional/terminal/`.

- `alter_slashes` belongs in `testutil.lua`, not `testnvim.lua`.
- `alter_slashes` is an unusual name. Rename it to `fix_slashes`.
- invert its behavior, to emphasize that `/` slashes are the preferred,
  pervasive convention, not `\` slashes.