feat: add Grammar2 construct support for TypedField, TypedVariant, TypedParameter, TypedFieldInit

- Add binding scope to execute_rule to prevent nested rules from overwriting parent bindings
- Add TypedDeclaration::Class support for struct declarations
- Add TypedField construct and ParsedValue::Field variant
- Add TypedVariant construct for enum variants
- Add TypedParameter construct for function parameters
- Add TypedFieldInit construct for struct literal field initializations
- Add enum constant handlers for TypedVariantFields::Unit, ParameterKind::*, Mutability::*
- Add TypedExpression::Literal and TypedLiteral variant handlers

Author: darmie