Remove development and test files from the gem package

[orien]

There are several files in the gem package that aren't useful for downstream projects. Removing these files reduces the gem package size from 34K to 24K!

gem contents diff

```diff < .github/workflows/ci.yml < .gitignore CHANGES.md < Gemfile README.md < Rakefile < TODO.md cells.gemspec lib/cell.rb lib/cell/abstract.rb lib/cell/builder.rb lib/cell/caching.rb lib/cell/collection.rb lib/cell/concept.rb lib/cell/development.rb lib/cell/escaped.rb lib/cell/inspect.rb lib/cell/layout.rb lib/cell/option.rb lib/cell/partial.rb lib/cell/prefixes.rb lib/cell/self_contained.rb lib/cell/templates.rb lib/cell/testing.rb lib/cell/twin.rb lib/cell/util.rb lib/cell/version.rb lib/cell/view_model.rb lib/cells.rb lib/tasks/cells.rake < test/builder_test.rb < test/cache_test.rb < test/cell_benchmark.rb < test/cell_test.rb < test/concept_test.rb < test/context_test.rb < test/fixtures/bassist/play.erb < test/fixtures/cell_test/song/show_with_block.erb < test/fixtures/comment/layout/show.erb < test/fixtures/comment/show/show.erb < test/fixtures/concepts/record/views/layout.erb < test/fixtures/concepts/record/views/show.erb < test/fixtures/concepts/record/views/song.erb < test/fixtures/inherit_views_test/popper/tap.erb < test/fixtures/inherit_views_test/tapper/play.erb < test/fixtures/inherit_views_test/tapper/tap.erb < test/fixtures/partial_test/with_partial/show.erb < test/fixtures/partials/_show.html.erb < test/fixtures/partials/_show.xml.erb < test/fixtures/song/ivar.erb < test/fixtures/song/show.erb < test/fixtures/song/with_block.erb < test/fixtures/song/with_erb.erb < test/fixtures/song/with_html.erb < test/fixtures/song/with_locals.erb < test/fixtures/song_with_layout/happy.erb < test/fixtures/song_with_layout/merry.erb < test/fixtures/song_with_layout/show.erb < test/fixtures/song_with_layout/show_with_layout.erb < test/fixtures/templates_caching_test/song/show.erb < test/fixtures/url_helper_test/song/edit.erb < test/fixtures/url_helper_test/song/with_block.erb < test/fixtures/url_helper_test/song/with_capture.erb < test/fixtures/url_helper_test/song/with_content_tag.erb < test/fixtures/url_helper_test/song/with_form_for_block.erb < test/fixtures/url_helper_test/song/with_link_to.erb < test/inspect_test.rb < test/layout_test.rb < test/partial_test.rb < test/prefixes_test.rb < test/property_test.rb < test/public_test.rb < test/render_test.rb < test/templates_test.rb < test/test_helper.rb < test/testing_test.rb < test/twin_test.rb ```