Add change_column_null from BulkChangeTable

[fkmy]

Is your feature request related to a problem? Please describe.

There is a cop BulkChangeTable that checks for combinable alter queries when changing multiple columns.

Sorry if I am mistaken, but I believe that in the past there was no corresponding change_table method for the change_column_null method, and that is how it was removed. #48

But then the change_null method in change_table is added, which corresponds to the change_column_null method.
rails/rails@972d18e

Describe the solution you'd like

I want to check for combinable alter queries and warn if NULL constraint changes are made to multiple columns.

# bad
def change
  change_column_null :users, :name, false
  change_column_null :users, :nickname, false
end

# good
def change
  change_table :users, bulk: true do |t|
    t.change_null :name, false
    t.change_null :nickname, false
  end
end

Describe alternatives you've considered

Since there is a method change_null in change_table corresponding to the change_column_null method, how about adding a change check for null constraints to BulkChangeTable?

Additional context

Nothing specifically.