refactor the naming within plugin new generator

Having a method called `full?`, which checks on :full and :mountable
is very confusing. I renamed `full?` to `engine?` and created a `full?`
method that only checks the `:full` option
remove meaningless AS::FrozenObjectError
Merge pull request #8682 from amatsuda/rm_as_frozen_object_error

remove meaningless AS::FrozenObjectError
Add missing changelog entry for #8676 [ci skip]
Merge pull request #8686 from mekishizufu/changelog_entry

Add missing changelog entry for #8676 [ci skip]
Make content_tag_for work without block

This is version of #8640 for master
No need to pass a empty block to content_tag_for anymore
allow toggling dumps instead of just showing
add source line padding
Merge pull request #8689 from goshakkk/error-page

Add source code left padding on error pages
refactoring to_sym of Symbol in preload
Cleanup some unnecessary CSS on the new error page and reformat some lines.
Merge pull request #8691 from acapilleri/preload_association

refactoring to_sym of Symbol in preload
replace #flatten with Array()
Merge pull request #8692 from lucasmazza/error-page-css

Cleanup the CSS on the new error page.
Merge pull request #8693 from goshakkk/flatten

Replace #flatten with Array()
remove unnecessary #dup
Merge pull request #8688 from goshakkk/error-page-toggle

Allow toggling dumps on error page
Merge pull request #8694 from goshakkk/undup

Remove unnecessary #dup
remove another unnecessary dup