Changes type system so builtins and custom types share a sequential pool
of indices. Builtins (Void, Node, String) are emitted first if used,
then custom types follow - no reserved slots for unused builtins.

Key changes:
- Add Void, Node, String variants to TypeKind enum
- Remove QTypeId::VOID/NODE/STRING constants and is_builtin() method
- Update TypeTableBuilder to emit builtins as TypeDefs when referenced
- Update bytecode dump to use dynamic index width formatting
- Update TypeScript emitter to use TypeKind checks instead of constants

This eliminates index gaps in bytecode (e.g., T0, T1, T3 → T0, T1, T2).