Work on whitespace for find_node_at_offset_with_descend

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

Start at AssistContext::find_node_at_offset_with_descend and reproduce the macro example with the offset on whitespace. Trace descended cases to ensure they contain the whitespace being queried; done means whitespace returns None and does not descend into unrelated cases such as WildcardPat.