checker: check struct reference fields uninitialized (fix #19559)

[yuyi98]

This PR check struct reference fields uninitialized (fix #19559).

• Check struct reference fields uninitialized.
• Add test.

struct Viewport {
mut:
	parent &Window
}

pub struct Window {
mut:
	desktop_viewport ?&Viewport = none
	phone_viewport   ?&Viewport = none
}

fn main() {
	_ := &Window{}
}

PS D:\Test\v\tt1> v run .
tt1.v:8:32: warning: unnecessary default value of `none`: struct fields are zeroed by default
    6 | pub struct Window {
    7 | mut:
    8 |     desktop_viewport ?&Viewport = none
      |                                   ~~~~
    9 |     phone_viewport   ?&Viewport = none
   10 | }
tt1.v:9:32: warning: unnecessary default value of `none`: struct fields are zeroed by default
    7 | mut:
    8 |     desktop_viewport ?&Viewport = none
    9 |     phone_viewport   ?&Viewport = none
      |                                   ~~~~
   10 | }
   11 |
tt1.v:13:8: error: reference field `Window.desktop_viewport.parent` must be initialized (part of struct `Viewport`)
   11 | 
   12 | fn main() {
   13 |     _ := &Window{}
      |           ~~~~~~~~
   14 | }
tt1.v:13:8: error: reference field `Window.phone_viewport.parent` must be initialized (part of struct `Viewport`)
   11 | 
   12 | fn main() {
   13 |     _ := &Window{}
      |           ~~~~~~~~
   14 | }