parser, fmt: fix formatting interface method with pre-comments (related #18980)

[yuyi98]

This PR fix formatting interface method with pre-comments (related #18980).

• Fix formatting interface method with pre-comments.
• Add test.

vlib\v\fmt\tests\interface_method_with_pre_comments_keep.vv

module main

fn main() {
}

pub interface IOperateResult {
mut:
	// is_success
	is_success bool
	// is_show_msg
	is_show_msg bool
	// msg
	msg string
	// comment before method def
	set_msg(msg string) // comment after method def
}