Problem:
_extui module name is confusing and should eventually end up in _core/.

Solution:
Move it there and name it ui2.

Problem:  The wildmenu is hidden behind the dialog window with "list" in 'wildmode'.
          Global ('laststatus' set to 3) statusline is hidden behind the
          pager window.
Solution: Check wildmenumode() to adjust the dialog position when necessary.
          Ensure pager is positioned above the global statusline with 'laststus' set to 3.

Problem:  Fast context for msg_show event inhibits vim.ui_attach from
          displaying a stream of messages from a single command.

Solution: Remove fast context from msg_show events emitted as a result
          of explicit API/command calls. The fast context was originally
          introduced to prevent issues with internal messages.

Problem:
Trying to match the search highlight groups to the Normal highlight for
the window can fail when the message highlighting contains a fg/bg that
the Normal highlight doesn't (like an error message in cmd will have
ErrorMsg highlight instead of MsgArea - which is Normal in cmd.)

Solution:
Link the search highlight groups to an empty group in 'winhighlight'
thus disabling them instead of overriding them with Normal/MsgArea/etc.

fix(ui2): hide search highlights in msg window.

Problem:  Search highlighting is shown in the msg (and dialog) window.
Solution: Hide search highlighting in all but the pager window.

Problem:  Resize events during startup may clear an active external
          cmdline, which is then not redrawn.
          UI2 VimResized autocommand does not work.
          UI2 message appearance may be altered by inherited window
          options. The message separator uses the wrong fillchar.

Solution: Unset cmdline_was_last_redrawn when clearing the screen, such
          that cmdline_show is re-emitted.
          Ensure set_pos function is called without arguments.
          Ensure such options are unset. Use 'fillchars'->msgsep.

Problem: Setting a filetype before configuring default options for ui2
buffers (pager, cmd, ...) prevents users from setting their own options.

Solution: Call nvim_set_option_value after defaults are set.

Closes #36314

Co-authored-by: Luuk van Baal <luukvbaal@gmail.com>

fix(extui): set options and buffer name after 'eventignorewin'

Problem:  Setting up extui buffers emits OptionSet and BufFilePost events.
Solution: Set options and buffer name after 'eventignorewin'.

Problem:  Unable to see e.g. `inputlist()` prompts that exceed the dialog
          window height.
          Multi-line prompts are not handled properly, and tracking
          is insufficient for messages in cmdline_block mode.
Solution: Add vim.on_key handler while the dialog window is open that
          forwards paging keys to the window.
          Properly render multi-line prompts. Keep track of both the start
          and end of the current cmdline prompt. Append messages after the
          current prompt in cmdline_block mode.