Support braceless shape types in T::Struct fields

[Morriar]

Summary

Depends on #645.

Add support for braceless hash-rocket shape types in T::Struct fields:

class Configuration < T::Struct
  const :settings, "timeout" => Integer
  prop :options, :enabled => T::Boolean
end

The parser now restores the braces required to represent these values unambiguously as types:

class Configuration < T::Struct
  const :settings, {"timeout" => Integer}
  prop :options, {:enabled => T::Boolean}
end

• support string, symbol, mixed, and multiple shape keys in const and prop
• move shared shape normalization into Parser::Visitor
• reuse the same normalization for returns, top-level bind, and T::Struct fields
• preserve keyword argument syntax such as prop :options, enabled: T::Boolean rather than treating it as a shape
• translate normalized fields into valid RBS attribute and constructor types