As discussed f2f.

• Remove LeadingTrivia and TrailingTrivia non-terminals.
• Include the underlying trivia as siblings to the other rules/tokens in the parent directly.
• Use FieldName::LeadingTrivia and FieldName::TrailingTrivia for the promoted siblings.
• Remove fn is_trivia() for RuleKind, since it is impossible for any variant.
• Implement fn is_trivia() correctly for TokenKind based on Item::Trivia, instead of returning false. (Fix parsing ambiguous call options #862)