Avoid generating syntax errors for capitalized protobuf fields

[Morriar]

Motivation

With Protobuf it seems possible to write fields with capitalized names:

add_message "MyCart" do
  optional :ShopID, :int32, 1
  optional :ShopName, :string, 2
end

In its current state, Tapioca will generate a syntactically incorrect RBI:

class Cart
  sig { params(ShopID: String, ShopName: String).void }
  def initialize(ShopID: nil, ShopName: nil); end # Syntax error: formal argument cannot be a constant

  # ...
end

While we do want to generate constructors with named fields, we could fallback on taking a Hash if the name of one of the field is not suitable for a named parameter:

class Cart
  sig { params(fields: T.untyped).void }
  def initialize(**fields); end
end

Tests

See automated tests.