Rename AddColumnWithDefault to ReversibleAddColumnWithDefault

We're going to add another cop that deals with another aspect of
`add_column_with_default`, so we need to separate them.

rubocop/cop/migration/reversible_add_column_with_default.rb

@@ -5,7 +5,7 @@ module RuboCop
     module Migration
       # Cop that checks if `add_column_with_default` is used with `up`/`down` methods
       # and not `change`.
-      class AddColumnWithDefault < RuboCop::Cop::Cop
+      class ReversibleAddColumnWithDefault < RuboCop::Cop::Cop
         include MigrationHelpers
 
         def_node_matcher :add_column_with_default?, <<~PATTERN

rubocop/rubocop.rb

@@ -1,9 +1,9 @@
 require_relative 'cop/custom_error_class'
 require_relative 'cop/gem_fetcher'
 require_relative 'cop/migration/add_column'
-require_relative 'cop/migration/add_column_with_default'
 require_relative 'cop/migration/add_concurrent_foreign_key'
 require_relative 'cop/migration/add_concurrent_index'
 require_relative 'cop/migration/add_index'
 require_relative 'cop/migration/remove_concurrent_index'
 require_relative 'cop/migration/remove_index'
+require_relative 'cop/migration/reversible_add_column_with_default'

spec/rubocop/cop/migration/reversible_add_column_with_default_spec.rb

@@ -3,9 +3,9 @@ require 'spec_helper'
 require 'rubocop'
 require 'rubocop/rspec/support'
 
-require_relative '../../../../rubocop/cop/migration/add_column_with_default'
+require_relative '../../../../rubocop/cop/migration/reversible_add_column_with_default'
 
-describe RuboCop::Cop::Migration::AddColumnWithDefault do
+describe RuboCop::Cop::Migration::ReversibleAddColumnWithDefault do
   include CopHelper
 
   subject(:cop) { described_class.new }