Implement todo option

[ksss]

The todo option excludes predefined output from prototype runtime.

Issue

#1449

Supported features

This PR is implemented to exclude the following.

• Instance method
• Singleton method
• Constant
• Mixin

Demo

$ bundle exec rbs prototype runtime --todo IO
class IO
  public

  def external_encoding: () -> untyped

  def pread: (*untyped) -> untyped

  def pwrite: (untyped, untyped) -> untyped

  def to_path: () -> untyped

  def wait_priority: (*untyped) -> untyped
end

[soutaro]

Thanks!