Consistent CSS class naming

[mathisonian]

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Describe the bug

The components defined by idyll-components declare CSS class names for components, which are then matched to queries in the theme and layout.

These class names do not follow a consistent style. They should all be updated to be prefixed by idyll-, so e.g. className="article-header" becomes className="idyll-article-header". The CSS selectors in the themes and layouts files should be updated, where appropriate, to match.

Expected behavior The behavior should remain unchanged, but CSS rules should be updated to use a more consistent style.

Additional context

This is relevant to https://github.com/idyll-lang/idyll/issues/494

[mathisonian]

It would be good to get this in with v5, since it will be a breaking change.