Support forwarding parameters in method types

[Morriar]

Summary

Add parser and AST support for Ruby-style forwarding parameters in RBS method types.

def request: (...) -> Response

def logged_request: (String message, ...) -> Response

RBS already uses ... to retain existing overloads:

def foo: (Integer) -> String
       | ...

The two forms are unambiguous because forwarding appears inside a parameter list, while overload continuation appears where a method type is expected.

Implementation

Represent forwarding as an optional RBS::Types::Function::ForwardingParam node:

function.forwarding # RBS::Types::Function::ForwardingParam | nil

The node preserves the location of ... for diagnostics and syntax-aware tooling:

function.forwarding.location.source # => "..."

The change updates:

• The C parser and generated AST
• Native and WASM AST translation and serialization
• Ruby types and RBS declarations
• JSON schemas

Syntax restrictions

Forwarding must be the final parameter and may only follow required positional parameters.

Accepted:

(...) -> void
(String message, ...) -> void

Rejected:

(?String message, ...) -> void
(*String args, ...) -> void
(message: String, ...) -> void
(**String kwargs, ...) -> void
(..., String) -> void

Forwarding is also rejected in Proc and block parameter types:

^(...) -> void
() { (...) -> void } -> void

Because forwarding includes the block, an additional explicit block is rejected:

(...) { () -> void } -> void
(...) ?{ () -> void } -> void

The existing overload-continuation syntax continues to work alongside forwarding:

def foo: (...) -> void
       | ...