Work on whitespace for find_node_at_offset_with_descend

[A4-Tacks]

AssistContext::find_node_at_offset_with_descend Return None on the whitespace within the macro, as the whitespace does not have descended

macro_rules! m { ($($t:tt)*) => {$($t)*} }
m!(match Some(2) {
    None => 3,
    $0
});

$0 cannot descend a token on WHITESPACE, unless $0 is on the }

Related #21453

Problem

Avoid descended cases that do not contain the this whitespace, e.g
call find_node_at_offset_with_descend::<ast::WildcardPat>() on $0 _ => 2

[ChayimFriedman2]

This is difficult. A simple solution is to look at the neighbor token if we're descending trivia. It's hard to decide which neighbor though, different contexts may have different needs in this.