checker: fix mismatch checking when a function returns sumtype as an argument(fix #19325)

[shove70]

1. Fixed Typecasting / -checking for callback return sumtypes does not work #19325
2. Add tests.

type Response = int | string

fn my_resp() string {
	return 'hello'
}

fn event(cb fn () Response) {
	resp := cb()
	dump(resp is string)
}

event(my_resp)

outputs:

a.v:12:7: error: cannot use `fn () string` as `fn () Response` in argument 1 to `event`
   10 | }
   11 | 
   12 | event(my_resp)
      |       ~~~~~~~
   13 | 
   14 |

[shove70]

😆😆😆 Very hacky indeed...... hahahaha

I'll change it to this later:

zz.v:13:7: error: cannot use `fn (string) string` as `fn (Response) Response` in argument 1 to `event`
   11 | }
   12 |
   13 | event(myfn)
      |       ~~~~

[spytheman]

Excellent work.