[Proposal] Make`-> void` return type annotation on `initialize` methods to be implicit

I propose simplifying the RBS syntax for initialize methods by removing the required -> void return type annotation.

Current Syntax:

#: (a: String) -> void
def initialize(a:)
  @a = a
end

Proposed Syntax:

#: (a: String?)
def initialize(a:)
  @a = a
end

Why?

1. Redundant - I was moving from Sorbet sigs to RBS style sigs and noticed that I was adding the -> void to every initialize method. Even though it makes sense from a completeness perspective, it seemed redundant to me and does not add any new information.

2. Initialize is special - The initialize method is already special in Ruby - it's the only method that is called indirectly through new, cannot meaningfully return a value to its caller, has a fixed, unchangeable return behavior. Making its type signature special in RBS would accurately reflect its special status in the language.

3. Comparison with other languages:

   Language	Example	Requires Return Type?
   Python (with types)	def __init__(self, a: str):	Optional
   TypeScript	constructor(a: string) { }	No
   Java	public MyClass(String a) { }	No
   C#	public MyClass(string a) { }	No
   Go	N/A - uses factory functions instead	N/A
   Ruby (current RBS)	#: (a: String) -> void	Yes

The issue names no files or tests. Start by locating the RBS parser and validation paths for initialize method signatures, then compare the current and proposed syntax examples. Done requires an agreed design for the special return-type rule, with its behavior documented and covered by appropriate tests.