[Wildcard Variables] Support for wildcard record type positional fields

[pq]

typedef R = (String _, String _);
(int _, int _) record;

It is currently an error for a (positional) record field name to begin with _ (including just a bare _). We relax that error to only apply to record fields whose name begins with _ followed by at least one other character (even if those later character(s) are _).

Named fields of record types are unchanged. It is still a compile-time error for a named field name to start with _.

https://github.com/dart-lang/language/blob/main/working/wildcards/feature-specification.md#record-type-positional-fields

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• update INVALID_FIELD_NAME_PRIVATE to allow positional _s in record types.
• update DUPLICATE_FIELD_NAME to allow multiple positional _s in record types