ast, parser, fmt: fix formatting struct declaration with comments (fix #18982)

[yuyi98]

This PR fix formatting struct declaration with comments (fix #18982).

• Fix formatting struct declaration with comments.
• Add test.

vlib\v\fmt\tests\struct_decl_with_comments_keep.vv

module main

fn main() {
}

pub struct OperateInfo { // implements IperateInfo
pub mut:
	title string // title
	// msg
	msg string
	// id
	id string
} // operate info

[spytheman]

Good work.