Allow consumers to define styles by default

[ijlee2]

Background

In 0.4.0, the styles for the collapsible case affect those for the default case. As a result, the docs for ember-intl will show a top border when the following code is rendered:

<CodePreview src="/components/locale-switcher.gts">
  <CodePreview src="/snippets/helpers/format-date/example-1/component.gts" />
</CodePreview>

This pull request, based on ember-intl's patch, removes styles for the default case so that consumers can decide for themselves. It also renames the class selector ember-playground__source to ember-playground__show-code.