cgen: fix struct field option type with default value init (fix #19418)

[yuyi98]

This PR fix struct field option type with default value init (fix #19418).

• Fix struct field option type with default value init.
• Add test.

type SomeType = SomeStruct | string

struct SomeStruct {}

struct AnotherStruct {
	field ?SomeType = 'default_string'
}

fn main() {
	s := AnotherStruct{}
	println(s.field)
	assert '${s.field}' == "Option(SomeType('default_string'))"
}

PS D:\Test\v\tt1> v run .
Option(SomeType('default_string'))