checker: check struct init with pointer field (fix #18485)

[yuyi98]

This PR check struct init with pointer field (fix #18485).

• Check struct init with pointer field.
• Add test.

struct Example {
	example voidptr
}

fn main() {
	println(Example{
		example: get()
	})
}

fn get() string {
	return 'hello'
}

PS D:\Test\v\tt1> v run .
tt1.v:7:3: error: cannot assign to field `example`: expected a pointer `voidptr`, but got `string`
    5 | fn main() {
    6 |     println(Example{
    7 |         example: get()
      |         ~~~~~~~~~~~~~~
    8 |     })
    9 | }