checker: check expr evaluated but not used (fix #21436)

[yuyi98]

This PR check expr evaluated but not used (fix #21436).

• Check expr evaluated but not used.
• Add test.

fn main() {
	{1, 2, 3, 4}
	println('works')
}

PS D:\Test\v\tt1> v run .
tt1.v:2:3: error: expression evaluated but not used
    1 | fn main() {
    2 |     {1, 2, 3, 4}
      |      ^
    3 |     println('works')
    4 | }