Add useUserAgent

[Tommypop2]

• Added useUserAgent primitive, which allows for the user to retrieve the user agent string on the client or the server
• Restructured code slightly, so that different start primitives are located within different files

[changeset-bot]

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Latest commit: ffe8a97

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[Tommypop2]

This PR is quite old now and relies on old start, so shouldn't be merged in its current state. I'll happily update it to new start if there's still interest in this functionality though!