Implement get_length() for pipes.

[bruvzg]

• Fixes get_line() getting stick if used with the pipes.
• Add implementation of get_length for Windows and Unix pipes, allows checking for available input buffer bytes:

if pipe["stdio"].is_open():
	var sz = pipe["stdio"].get_length()
	if sz > 0:
		var buffer = pipe["stdio"].get_buffer(sz)
		$TextEdit.text += buffer.get_string_from_utf8()

Fixes #102340

[Repiteo]

Thanks!