@fluent/syntax 0.17.1

• Align AST types with fluent-rs. (#525)
  Use Union types instead of inheritance for Entry, PatternElement, InlineExpression, Expression, Literal, and Comments. The union types allow for type narrowing in TypeScript consumers.
• Use concrete argument types for Visitor and Transformer. (#525)
• Update TypeScript to 4.2 to support type imports and exports.