feat(tui): add nvim_ui_send (#35406)

This function allows the Nvim core to write arbitrary data to a TTY
connected to a UI's stdout.

test/functional/ui/screen.lua

@@ -48,6 +48,7 @@
 local t = require('test.testutil')
 local n = require('test.functional.testnvim')()
 local busted = require('busted')
+local uv = vim.uv
 
 local deepcopy = vim.deepcopy
 local shallowcopy = t.shallowcopy
@@ -89,6 +90,7 @@ end
 --- @field private _grid_win_extmarks table<integer,table>
 --- @field private _attr_table table<integer,table>
 --- @field private _hl_info table<integer,table>
+--- @field private _stdout uv.uv_pipe_t?
 local Screen = {}
 Screen.__index = Screen
 
@@ -235,6 +237,7 @@ function Screen.new(width, height, options, session)
       col = 1,
     },
     _busy = false,
+    _stdout = nil,
   }, Screen)
 
   local function ui(method, ...)
@@ -278,6 +281,12 @@ function Screen:set_rgb_cterm(val)
   self._rgb_cterm = val
 end
 
+--- @param fd number
+function Screen:set_stdout(fd)
+  self._stdout = assert(uv.new_pipe())
+  self._stdout:open(fd)
+end
+
 --- @param session? test.Session
 function Screen:attach(session)
   session = session or get_session()
@@ -1416,6 +1425,12 @@ function Screen:_handle_msg_history_show(entries, prev_cmd)
   self.msg_history = { entries, prev_cmd }
 end
 
+function Screen:_handle_ui_send(content)
+  if self._stdout then
+    self._stdout:write(content)
+  end
+end
+
 function Screen:_clear_block(grid, top, bot, left, right)
   for i = top, bot do
     self:_clear_row_section(grid, i, left, right)