[Wildcard Variables] consider a use_wildcards lint

[pq]

Not crazy about the name so other ideas welcome, but the idea is simply to introduce a lint that flags places where folks are using multiple underscores where they could be using a wildcard variable.

For example:

f(int _ , int __) { }

g() {
  var _ = h();
  var __ = h();
}

@useResult
int h() => ...;

#59499 and #59491 propose changing existing lints to do this nudging but after some thinking and conversation, I'm no longer convinced that we want to do this in either. Some reasons:

1. a targeted lint would allow for better targeted documentation
2. a targeted lint could get broader adoption (neither no_leading_underscores_for_local_identifiers nor non_constant_identifier_names are enabled in Google3 for example)
3. Number 2 is especially significant since having an enabled lint in Google3 will greatly help with migration but enabling either no_leading_underscores_for_local_identifiers or non_constant_identifier_names isn't an option because of false positives.

/fyi @kallentu @bwilkerson

[pq]

Another alternative is updating no_wildcard_variable_uses to do this nudging (as suggested in #59470).

/cc @natebosch @lrhn

[pq]

Closing in favor of the meta conversation in #56595