add Production::inlined: bool flag

[OmarTawfik]

Requirements

• Prevent it from generating its own RuleKind or TokenKind or ProductionKind.
• It will behave as if it was inlined into the referencing productions, but without a name.

Missing Validation

• Parser defs cannot reference inlined Scanner defs.
• Scanner defs can reference only inlined Scanner defs, except in not: and subtrahend:.
• Inlined Scanner defs cannot reference top-level Scanner defs.
• Only Trivia can reference inlined Trivia.