checker: add check if as cast used incorrectly for selector-expr

[Delta456]

Adds a checker error as proposed at #20718 (comment)

vlib/v/checker/tests/as_cast_selector_expr_err.vv:15:14: error: indeterminate `as` cast, use parenthesis to clarity
   13 | fn main() {
   14 |     mut bar := Foobar(Bar{y: 123})
   15 |     println(bar as Bar.y == 123)
      |                 ~~
   16 | }
Details: for example `(bar as main.Bar).y`

[spytheman]

Excellent work.