add column_type method to Models

sometimes its useful for creating reusable code.

It used to be available as an undocumented method in rails FYI...

```ruby
module ActiveRecord
if RUBY_ENGINE == 'opal'
class Base
# datetime, date, boolean, float, integer, string
def column_type(attr)
backing_record.column_type(attr)
end
def self.column_type(attr)
ReactiveRecord::Base.column_type(columns_hash[attr])
end
end
end
end
```