Problem:  `cmdline_show` is emitted unnecessarily each event
          loop iteration, because `cmdline_was_last_drawn` is never set.
Solution: Keep track of whether the cmdline was last drawn to avoid
          unnecessarily emitting cmdline_show. Set `redraw_state` to
          emit `cmdline_pos` when emitting `CursorMovedC`. Only emit
          `cmdline_pos` when cmdline was last drawn.

Problem:  nlua_call_ref_ctx() does not pop the return value in fast
          context that did not error.
Solution: Fall through to end; calling nlua_call_pop_retval().

Problem:  If a (vim.ui_attach) cmdline_hide callback triggers a redraw,
          it may cause cmdline_show events for an already exited cmdline.
Solution: Avoid emitting cmdline_show event when ccline.cmdbuff is
          already NULL. Unset ccline.cmdbuff before emitting cmdline_hide.

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:  Prompts are emitted as messages events, where cmdline events
          are more appropriate. The user input is also emitted as
          message events in fast context, so cannot be displayed with
          vim.ui_attach().
Solution: Prompt for user input through cmdline prompts.

Pass along whether message in msg_show event is added to the internal
:messages history.

Problem:  Unable to tell what highlight the prompt part of a
          cmdline_show event should have, and whether cmdline_hide was
          emitted after aborting.
Solution: Add additional arguments hl_id to cmdline_show, and abort to
          cmdline_hide.

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.

Add kinds for various commands that output a list, the 'wildmode'
list, and for number prompts.