Add `@_UncheckedMemberwiseInit` macro (#36)

[gohanlon]

Introduce a new experimental macro for generating memberwise initializers with reduced safety checks compared to @MemberwiseInit.

Features of @_UncheckedMemberwiseInit:

• Include all properties in the initializer, regardless of access level
• Include attributed properties by default (differs from @MemberwiseInit)
• Allow exposure of lower access level members without per-member annotation
• Has the same usage as @MemberwiseInit

@_UncheckedMemberwiseInit provides a trade-off between ease of use and compile-time safety, suitable for scenarios where brevity is preferred over strict access control enforcement. Note that the underscore prefix indicates this is an experimental feature.

Example:

@_UnsafeMemberwiseInit(.public)
public struct ViewModel {
  private let title: String
}

Yields:

public init(title: String) {
  self.title = title
}