feat(ui): add chdir UI event (#27093)

When an embedded Nvim instance changes its current directory a "chdir"
UI event is emitted. Attached UIs can use this information however they
wish. In the TUI it is used to synchronize the cwd of the TUI process
with the cwd of the embedded Nvim process.

test/functional/ui/screen.lua

@@ -140,6 +140,7 @@ function Screen.new(width, height)
     suspended = false,
     mode = 'normal',
     options = {},
+    pwd = '',
     popupmenu = nil,
     cmdline = {},
     cmdline_block = {},
@@ -212,7 +213,6 @@ function Screen:attach(options, session)
   if options.ext_linegrid == nil then
     options.ext_linegrid = true
   end
-
   self._session = session
   self._options = options
   self._clear_attrs = (not options.ext_linegrid) and {} or nil
@@ -1108,6 +1108,10 @@ function Screen:_handle_option_set(name, value)
   self.options[name] = value
 end
 
+function Screen:_handle_chdir(path)
+  self.pwd = vim.fs.normalize(path, { expand_env = false })
+end
+
 function Screen:_handle_popupmenu_show(items, selected, row, col, grid)
   self.popupmenu = { items = items, pos = selected, anchor = { grid, row, col } }
 end