chore: refactor client build data

[benmccann]

A first step for #3882. It's easier to deal with imports, stylesheets, and fonts than app.imports, app.stylesheets, app.fonts, start.imports, start.stylesheets, and start.fonts. This will become more true when we potentially have only a single entry point, so it makes the interface more generic and easier to deal with in that case.

[changeset-bot]

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