v: Fix concrete fn type that returns pair type as generic type

[ChAoSUnItY]

• Fix v: concrete pair type cannot use as generic pair type #17778.
• Add tests

concrete_type_as_generic_type_test.v

// ...

fn func_fn_multi_return_concrete() FnMultiReturn[string, string, string] {
	return fn (s string) (string, string) {
		return s[..1], s[1..]
	}
}

fn test_concrete_function_type_as_generic_type() {
	// ...
	s1, s2 := func_fn_multi_return_concrete()('VLang')

	assert s1 == 'V'
	assert s2 == 'Lang'
}

[spytheman]

Good work.