feat(ui): use builtin completion popupmenu with ext_cmdline (#31269)

Problem:  UIs implementing ext_cmdline/message must also implement
          ext_popupmenu in order to get cmdline completion with
          wildoptions+=pum.
Solution: Allow marking a window as the ext_cmdline window through
          nvim_open_win(), including prompt offset. Anchor the cmdline-
          completion popupmenu to this window.

runtime/doc/api.txt

@@ -3475,8 +3475,8 @@ nvim_open_win({buffer}, {enter}, {config})                   *nvim_open_win()*
                       like a tooltip near the buffer text).
                   • row: Row position in units of "screen cell height", may be
                     fractional.
-                  • col: Column position in units of "screen cell width", may
-                    be fractional.
+                  • col: Column position in units of screen cell width, may be
+                    fractional.
                   • focusable: Enable focus by user actions (wincmds, mouse
                     events). Defaults to true. Non-focusable windows can be
                     entered by |nvim_set_current_win()|, or, when the `mouse`
@@ -3558,6 +3558,9 @@ nvim_open_win({buffer}, {enter}, {config})                   *nvim_open_win()*
                     cursor will be invisible when focused on it.
                   • vertical: Split vertically |:vertical|.
                   • split: Split direction: "left", "right", "above", "below".
+                  • _cmdline_offset: (EXPERIMENTAL) When provided, anchor the
+                    |cmdline-completion| popupmenu to this window, with an
+                    offset in screen cell width.
 
     Return: ~
         |window-ID|, or 0 on error

runtime/lua/vim/_meta/api.lua

@@ -1771,8 +1771,7 @@ function vim.api.nvim_open_term(buffer, opts) end
 ---     - `row=0` and `col=0` if `anchor` is "SW" or "SE"
 ---       (thus like a tooltip near the buffer text).
 --- - row: Row position in units of "screen cell height", may be fractional.
---- - col: Column position in units of "screen cell width", may be
----          fractional.
+--- - col: Column position in units of screen cell width, may be fractional.
 --- - focusable: Enable focus by user actions (wincmds, mouse events).
 ---     Defaults to true. Non-focusable windows can be entered by
 ---     `nvim_set_current_win()`, or, when the `mouse` field is set to true,
@@ -1852,6 +1851,8 @@ function vim.api.nvim_open_term(buffer, opts) end
 ---         focused on it.
 --- - vertical: Split vertically `:vertical`.
 --- - split: Split direction: "left", "right", "above", "below".
+--- - _cmdline_offset: (EXPERIMENTAL) When provided, anchor the `cmdline-completion`
+---   popupmenu to this window, with an offset in screen cell width.
 --- @return integer # |window-ID|, or 0 on error
 function vim.api.nvim_open_win(buffer, enter, config) end
 

runtime/lua/vim/_meta/api_keysets.lua

@@ -312,6 +312,7 @@ error('Cannot require a meta file')
 --- @field noautocmd? boolean
 --- @field fixed? boolean
 --- @field hide? boolean
+--- @field _cmdline_offset? integer
 
 --- @class vim.api.keyset.win_text_height
 --- @field start_row? integer