Generate sigs for scope, has_many, etc.

[hdoan741]

It is not possible to generate the sigs right now because T.proc support is limited right now.
For example:
It doesn't work with generic sorbet/sorbet#1139
It doesn't work with variable inputs sorbet/sorbet#1142
T.proc.bind doesn't work with T.nilable sorbet/sorbet#498
T.proc.bind doesn't work with lambda sorbet/sorbet#1143

Once support is improved, we can generate scope it like this

  sig { 
    params(
      name: Symbol, 
      body: T.proc.bind(District::Relation).returns(T.untyped), 
      block: T.nilable(T.proc.void),
    ).void
  }
  def scope(name, body, &block); end

[dogweather]

Is this why I get, e.g.,

Method `has_many` does not exist on `T.class_of(...)`(7003)

?