support startup_message with narrow window size

In narrow console window, start up message shows like below:
```
+-----+
|Multi|
|line |
|REPL.|
|promp|
|t>   |
|     |
```
When start_tarminal() with ```startup_message: 'Multiline REPL.'```,
vterm.rb recieves startup message as stream, so window size is not matter.
windows.rb can't recieve startup message as stream, only can retreive_screen().

retreive_screen recieves like:
```["Multi", "line", "REPL.", "promp", "t>", "", ...]```

This patch splits startup_message into every width characters to allow for correct comparisons.

If there are wide widths or complex characters, they will fail to divide accurately.
So it is not perfect. Nevertheless, the situation can be mitigated.

lib/yamatanooroti/windows.rb

@@ -34,7 +34,7 @@ def start_terminal(height, width, command, wait: nil, timeout: nil, startup_mess
 
     case startup_message
     when String
-      wait_startup_message { |message| message.start_with?(startup_message) }
+      wait_startup_message { |message| message.start_with?(startup_message.chars.each_slice(width).map { |l| l.join.rstrip }.join("\n")) }
     when Regexp
       wait_startup_message { |message| startup_message.match?(message) }
     end