Move back `new_column_definition` into `TableDefinition`

Only `primary_key` should be extracted by d47357e2 in #19030, but
`new_coclumn_definition` was also extracted because #17631 is merged
previously, then #19030 is auto merged without conflicts.

This commit is for move back `new_column_definition` into
`TableDefinition`.

activerecord/lib/active_record/connection_adapters/abstract_mysql_adapter.rb

@@ -11,6 +11,10 @@ def primary_key(name, type = :primary_key, **options)
           options[:auto_increment] = true if type == :bigint
           super
         end
+      end
+
+      class TableDefinition < ActiveRecord::ConnectionAdapters::TableDefinition
+        include ColumnMethods
 
         def new_column_definition(name, type, options) # :nodoc:
           column = super
@@ -23,10 +27,6 @@ def new_column_definition(name, type, options) # :nodoc:
         end
       end
 
-      class TableDefinition < ActiveRecord::ConnectionAdapters::TableDefinition
-        include ColumnMethods
-      end
-
       class Table < ActiveRecord::ConnectionAdapters::Table
         include ColumnMethods
       end