cgen: fix printing struct with thread field (fix #19282)

[yuyi98]

This PR fix printing struct with thread field (fix #19282).

• Fix printing struct with thread field.
• Add test.

// Video is the object containing all the information needed for
// conversion to ascii.
struct Video {
pub:
	// path may be URL or path on local file system.
	path         string
	tmp_dir      string
	frame_count  int
	frame_format FrameFormat
pub mut:
	// TODO: once V fixes bug #19281 replace the type here
	// with `thread !`. And the return type on the corresponding
	// function that is spawned.
	extract_frames_thread thread string
}

enum FrameFormat {
	bmp
	jpg
}

fn (video Video) extract_frames() string {
	// try to extract frames os.execute()
	// if os.execute() returns with an exit
	// code other than 0, return an error.
	if true {
		return 'Failed to extract frames: os.execute() cmd output'
	}
	// do something else
	return ''
}

fn main() {
	mut video := Video{
		path: '/path/to/some/mp4'
		tmp_dir: '/tmp'
	}
	video.extract_frames_thread = spawn video.extract_frames()
	println(video)
}

PS D:\Test\v\tt1> v run .
Video{
    path: '/path/to/some/mp4'
    tmp_dir: '/tmp'
    frame_count: 0
    frame_format: bmp
    extract_frames_thread: thread(string)
}